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Why do we say "eat healthy" instead of "eat healthily", even though the latter is the only "correct" one, according to the comments in
"eat healthy" or "eat healthily"
What came first: ads saying "eat healthy", or the grammatical acceptability of such a phrase?
Merriam-Webster does not consider "healthy" to be an adverb, unlike "quick": https://www.merriam-webster.com/dictionary/healthy, and therefore this question is not a duplicate of As quick as we can?
Why would "eat healthy" be incorrect? This usage of "healthy" as an adverb is already in at least one major dictionary:
adv.
So as to promote one's health; in a healthy way: If you eat healthy, you'll probably live longer.
The American Heritage Dictionary
I'm not sure why other dictionaries are so slow to add this definition. It is used more often than "healthily". When I searched COCA, eat healthy _y* had 43 matches and eat healthily _y* had 4. (Note that _y* matches punctuation, to avoid matching healthy as an adjective. See this image for examples of what was matched in the eat healthy _y* search).
There are many adjectives that are used as flat adverbs (i.e. without "-ly"), even when there is a non-flat form. The paper "Love Me Tender": A Usage Study on Adverbs Which Keep Their Adjectival Form examines this.
It's only some prescriptivists that view these types of adverbs incorrect. Apparently the reasoning was originally based off Latin grammar:
As Emily Brewster notes in a short video about flat adverbs, prescriptive grammarians in the 18th century, being overly attached to Latin grammar, thought flat adverbs were really adjectives being used incorrectly, and warned against their use. Before this, flat adverbs were more common and varied than they are now. Exceeding is a good example. If we browse Daniel Defoe's writing we find such phrases as: weak and exceeding thirsty; it rained exceeding hard; it is exceeding confused. Today this usage has an archaic feel.
Flat adverbs are exceeding fine
For an example of how opinions differ, Garner's Modern American Usage is fine with flat adverbs, saying that both "drive slow" and "drive slowly" are correct. Other resources, such as the Purdue OWL believe that it's incorrect to use "adjectives" this way (saying that "ate her lunch quick" is incorrect). If you're using a style guide, be sure to check out what it says.
Advertisers are much more interested in coining a catchy, memorable slogan than in being grammatically correct. According to Oxford Dictionaries, in the article ‘The Real Thing’? The language of advertising slogans:
It is essential in a noisy and crowded world that a slogan stands out. Take the Apple slogan of the 1990s ‘Think Different’ – which used the adjective different rather than the expected (and more grammatically correct) adverb differently.
Note that this shows the same grammatical creativity as the OP's example eat healthy
Other famous examples of grammatical creativity in advertising:
Got Milk?
Winston tastes good like a cigarette should
All this is explained in Visual Thesaurus, in an article "Bad" Grammar in Ads: License to Annoy?
As someone who's spent my professional life in and around advertising, I can assure you that the writers responsible for these ads are in command of the English language, are fully aware of the rules, and broke them intentionally. Ad writers do this all the time: their goal, after all, is to make you stop and pay attention, and word play, word invention, and — yes — unconventional grammar are time-honored ways of accomplishing that end.
If you google grammatically incorrect advertising slogans you will find a rich literature on the topic.
