That's clearly clipped from the common construction as there are dope heads, hip-hop heads, meth heads and so on.
Accordingly, the English wiktionary defines head
(slang, countable) A heavy or habitual user of illicit drugs.
quoting from 1968, Fred Davis; Laura Munoz, “Heads and freaks: patterns and meanings of drug use among hippies”, in Journal of Health and Social Behavior, volume 9, number 2, page 156-64:
- The term, "head," is, of course, not new with hippies. It has a long history among drug users generally, for whom it signified a regular, experienced user of any illegal drug—e.g., pot "head," meth "head," smack (heroin) "head."
Despite this possible connotation, the definition is tacked on in the end (11.), appearing far removed from hip-hop heads (3.4.) and expertise (3.), leadership (2.), or the bodypart and its metonymies (1.1.5. "an individual person" e.g.)
This construction must be fairly old if it should be akin with German constructions i. Dummkopf, that is literally dope-head but chiefly in the sense "dumb, dope, stumped", ii. Quadratschädel, ie. "squarehead", Querkopf "blockhead", iii. adj. dickköpfig "thick headed".
Indeed, -head is noted as word forming morpheme in "pothead" ("1. Used to form (usually derogatory) words for people who regularly have their mind focused upon a particular subject, activity, or a specified drug or other substance, or who are addicted in some way.") and from this two competing subsense.
- Used to form words to describe people who are dedicated fans of something, especially music.
- Used with other words to form generic insults or epithets to indicate stupidity
blockhead, shithead, no-good geekhead
English is conservative whereas German has replaced inherrited haupt- for almost all purposes by borrowing "Kopf" from Latin caput, from the same root. Head shares the consonant of Grimm's law with Haupt; the -p- was lost separately, still present as affricate in Old English hēafod. Of course the root appears in forehead as well as headmaster and captain.
Dopehead makes sense no less under def. 1.1.2.
(figuratively, metonymically) Mind; one's own thoughts
- This song keeps going through my head
Compare the verb and adverb: mind sth., to be minded, drug minded.
PS: For a completely different take, consider coffee shop for comparison
- A small café or restaurant typically selling light refreshments along with coffee-based drinks.
Synonyms: café, coffeehouse, diner
- A café-like establishment selling marijuana in the Netherlands.
Dutch has ostensibly borrowed the second sense only from English, says English wiktionary: coffeeshop; at least the shop in koffieshop looks like it. In either case, coffe is well known to come from Arabic and Turkish traders, same for mokka. Nonetheless, the dark brew may be misleading for this kind of shop.
Ignore for a second the fact that the typical head shop was de legis not allowed--in most jurisdictions they still are not allowed--to sell scheduled substances over the counter. They would, however, if they could. The modern distinctions should not lead to etymological fallcies. Therefore,
- compare German kaufen "to buy" (as in Kaufman, surely via Yiddish), Dutch kopen, Old Danish Køpmannæhafn, En. Copenhagen, modern Danish København, "equivalent to modern Danish købmænds havn (“chapmen's haven, merchants' harbor”)." These may be reflexes of a Proto-Germanic word that's reconstructed *kaupô: "Borrowing from Latin caupō (“innkeeper, shopkeeper”),"
however, the further origins of this and that are uncertain.
cf. *káput "head" <? "Possibly from *kap- (“seize, hold”), perhaps of substrate origin.", q.v.:
"or perhaps onomatopoeic, compare *gʰabʰ- (“to seize”)".
"Alternative Reconstruction: *keh₂p-" -- Nearly equivalent to *kap-, Proto-Indo-European *eh₂ > *ā and *h₂e > *a are thought to be later developments in Laryngel theory different from the Proto-Indo-European *a *which is chiefly rejected by modern indoeuropeanists except for hypothetical substrate origins, onomatopeia, etc., cf. Tijmen Pronk (2019. Proto-Indo-European *a. doi: https://doi.org/10.1163/22125892-00701002; §§ 4.6 - 4.9)
*haven < *habanō < *kh₂póneh₂ or *kh₂pnéh₂, **kh₂pnós+ < *keh₂p- again reconstructs the same root, which doesn't even make sense, but see eg. beach head. -hagen has no clear explanation, but may be due to mismatching phonology (German SE: Why is there a g in “Copenhagen”?: Daish Havn [hau̯ˀn] (@takkat), "Nowadays the pronounciation of "g" is a major problem for those learning Danish - it is silent or pronounced by various kinds of guttural sounds, often very difficult to hear for a foreigner" (@KajRosling), possibly contaminated with Hag or Hain (@takkat), cp. The Hague. As per Pronk (§4.7), see also Haufen and the like. A similar approximant is also seen in Russian Копенга́ген (Kopengágen) beside eg. его́ (jevó, "him"), Гарри (Harry), and Western Romance, eg. Guilliam (William).
- Whereas English to cop "1. To obtain, to purchase (as in drugs), to get hold of, to take" (bold-emph. mine) is as uncertain as kopen as it may well derive therefrom from caupo or ultimately from Latin capiō (“to seize, grasp”) via French from the same more or less uncertain root.
Ancient Greek κεφαλή, which matches the sense of caput "head" perfectly, instead suggests *gʰebʰ- "to give", recently distinguished from *gʰeh₁bʰ- (*gʰabʰ- above, not even mentioned by Pronk). This may be a curious coincident along with to have vs. Lat. habeo "to have" from those respective roots, which should not concern us any further.
Latin caupo "inkeeper" and thus our kopen, too, is uncertain, either a substratal borrowing parallel to κᾰ́πηλος "retail dealer, huckster; innkeeper", compared unsuccessfully to *keh₂p- (cp. κάπη "crib, manger", "... the same root of κάπτω (káptō, “to gulp down”)") or from a root "to buy", or akin to copy shop, copio as derivations from co- + *h₃ep-, whence Latin ops "power", apprently unrelated to Greek ὄψ (óps, "eye; face")
German Kauf- was--and still is--productive as Bezugswort in the appellative lexicon, eg. Kauf-Halle (considered obsolete in the west), cp. valhalla.
German Schuppen itself may mean "tavern, club" rather than "shed, workshop", see also Frühschoppen, or perhaps -schaft "-ship, -hood" as in Gesellschaft "society, company", Geschäft "shop", geschäftig "worshipful", moreover joint, burger-joint (the phonology of j left as exercise for the reader, see chug "a large gulp of drink", or jag "variant of British English dialectal chag [...] from Proto-Germanic *kagô," for a start). Recall that skull and house, hut, hat seem to share, if not the same, at least very similar roots (see dome, temple for similar analogies to the forehead).
head, just as (Dutch) (kop)[https://en.wiktionary.org/wiki/kop#Dutch], has many more connotations than the dictionary can provide. If the head shop was for the longest time associated with shady business, nothing big, there is no imediate evidence of the word development to be expected. If it is with big business, on the other hand, loanwords and calques have to be reckoned with. Citations for earliest evidence would help, of course, but have to been omited to avoid the historians fallacy.
There is no doubt yet that the modern head shop, which sells exclusively marijuana related paraphernalia, advertised to heads--in the sense of let's say afficinados, mutatis mutandis. In view of all that, as nothing's really certain, head, coffee, and shop may be well connected much more closely than surface analysis would suggest. With the Dutch you never really now.