I was looking for the translation of the German word 'Schwarzarbeit' (black work) that means working illegally, without written contract, in order to avoid labour laws and taxation.
The Google Translator has proposed the word 'moonlighting', but from the context I know that word, it's more about doing illicit things like brewing alcohol without license.
Does 'moonlighting' have a (second) meaning that represents the meaning of 'Schwarzarbeit'?
While there used to be a criminal meaning for the word, that connotation is long gone:
Moonlight is the reflection of the sun off the moon's surface — a clear sky and a full moon provide brilliant moonlight. Smugglers hate moonlight. If you moonlight, you work a second job, and it doesn’t have to be at night. If your history teacher also works as a mascot for a baseball team, he moonlights as a dancing bear. As a verb, it used to mean “commit crimes at night,” but now just means “to work a second job.” — Vocabulary.com
Moonlighting is legal (with a few exceptions that aren't that important here).
Note: that's not the same as moonshine, which is typically a noun (but is sometimes used as a verb).
The phrase you are looking for is "work under the table":
To work in exchange for payment that is not officially documented and has not been taxed. — Farlex via TFD
That is illegal (as far as I know).
It originally referred to the Australian/Irish expression meaning to steal cattle; thus moonlighting noun.
It was later used to refer to:
(UK Und.) to engage in criminal activity at night.
- 1942 [US] Berrey & Van den Bark Amer. Thes. Sl.
- 1949 [US] Monteleone Criminal Sl. (rev. edn).
and to probably illegal but not criminal jobs.
(orig. US) to work at two jobs in order to boost one’s income. The second job is usu. night work, and the other employer may not know about it; thus more usual as the noun moonlighting.
1970 [US] E. Tidyman Shaft 35: Cops were moonlighting as armed cab drivers.
1978 [US] L. Kramer Faggots 116: He moonlighted, writing twice-monthly features.
(GDoS)
Moonlighting means a second job.
That's all it means.
This is remarkably unarguably, straightforwardly, stated in the Oxford English Dictionary:
verb (past and past participle moonlighted) [no object] informal have a second job in addition to one's regular employment: many instructors moonlight as professional consultants. - The OED.
For example, a very famous TV show (actually, the funniest TV comedy ever made in the US) was named Moonlighting - a fashion model ended up working in a completely different field, for comic reasons.
(Please note that on this site, extensive discussion about the origin of, or previous meanings of, a word - when the question is "What Does This Word Mean" - lead to confusion.)
Regarding Schwarzarbeit.
In English you simply say "working off the books" or "working for cash" or you might hear "cash in hand", "under the table", "off the books"; the phrase "the black economy" is used to describe that overall economy. There's no single term for working "cash in hand".
(Moonlighting is utterly unrelated and does not suggest it in any way.)
Others have given the correct denotation of "moonlighting" currently - a second job, one that possibly your main employer doesn't know about. For tax purposes, it definitely is different than being paid under the table (by your main or secondary employer) in that it's legal work and paid and taxed legally.
However, many employers take the same sort of dim view of "moonlighting" as the taxman does of "Schwarzarbeit", and if they find out about it, it will negatively impact your main job (overlooked for promotions, not given plum work, on the list for being found not performing to expectations: "he always seems so tired"). If you moonlight in the same field as you're being paid for, they may be in the right, or at least able to restrict it in the contract. If it's totally different, the company's actions may not be legal, but it happens anyway, and you'll never be able to prove anything; and companies where this attitude holds are ones where you really want to keep your moonlighting on as much of a lowdown as you would if your moonlighting job was illegal or being paid "under the table".
As Mari-Lou A and Ruadhan2300 mention in their comments to the OP, "moonlighting" implies not just working a second job, but keeping it secret from someone (usually your main employer or the public) - and frequently having a reason to (the policeman moonlighting as a drag queen, as mentioned in the comments, or someone working for an employer known to frown on "hobby jobs").
You indicated you were looking for a translation of Schwarzarbeit, meaning illegal “black work.” A close equivalent is “working on the black market,” which Merriam-Webster defines as
illicit trade in goods or commodities in violation of official regulations
Another possibility is working “under-the-table,” which the same dictionary defines as;
covert and usually unlawful
The connotation here is slightly different. Someone taking part in a black market is definitely exchanging illegal goods or services. Someone getting paid under the table could mean they’re doing an innocuous job, but (for example) their income is not being reported so as to evade taxes.
The colloquial word moonlighting (from 1954, originally U.S., moonlighter for the person doing it) for a secondary paid work (often at night) in addition to one's regular job is a given per current usage and dictionaries; however, the origin and the history of the word can give us a glare of its multifarious semantic development, helping us to make sense of the current meaning.
In fact, the current usage is a positive semantic shift (amelioration) as an analogy to its origin, illicit activities done at night (under the moonlight). The sense 'illicit activity' is apparently still there as OED adds that this usage is rare, possibly used in Irish English only, in some isolated cases. Here is the earlier second sense of moonlighting (from 1882) from OED:
The performance of an illicit action by night; (Irish History) the perpetration by night of raids on tenants who incurred the hostility of the Land League. Also figurative. Now rare.
Although, the earliest illicit activity sense, which is still in use, is moonlight flit; from 1824, defined in OED as:
n. the act of leaving one's accommodation without paying the rent; the removal of household goods by night to avoid paying rent; (hence) a hurried departure or escape by night.
However, OED also has moonlight flitting from as early as 1721.
n. Scottish (now rare) = moonlight flit n.
There is even a rare usage of moonlight (n.) meaning moonshine (.n) ('smuggled or illicitly distilled alcoholic liquor'), from 1809, according to OED; but adds that it is British regional.
Interestingly, there is an uncommon colloquial word sunlighting also (which I haven't heard before but found while searching), coined after moonlighting; from 1961, defined in OED as:
colloquial. The practice of having a second job in addition to one's regular employment, which is undertaken during the day (rather than at night).
Additional history:
There is also the earliest sense of the noun moonlighting listed in OED, from 1880, which appears to be originated independently in Australian English; defined in OED as:
Australian. The practice of mustering wild cattle at night.
In Australia, in old times, there was the practice of moonlighting, where they mustered wild cattle at night because they would come into clearing to graze at night and the stockmen needed sufficient moonlight to be able to see at night. Mustering is the act of collecting together wild cattle by riding round a scattered herd and driving it together. I've also found an Australian language book (by Sidney John Baker) talking about moonlighting as a method of mustering scrubbers (wild cattle) at night; and even as the shooting of possums seen in the tree branches against the light of the moon. The usage was not isolated to just one activity and has extended to other activities done in the moonlight. This tells us that all related senses are an extended sense from the light of the moon, moonlight.
In Ireland, a job that is not reported to the taxman is called a "nixer". Interestingly, the Irish revenue provides "moonlighting" as a synonym for nixer.
From https://www.revenue.ie/en/corporate/assist-us/reporting-shadow-economy-activity/nixers.aspx
What does the term 'nixer' mean in the context of tax and social welfare?
The term 'nixer' generally refers to part-time work that an individual undertakes and the income from which is not reported to relevant authorities such as Revenue.
