Etymology
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head shop (n.)

emporium for stoner gear, by 1969 (noted in 1966 as the name of a specific shop in New York City selling psychedelic stuff), from head (n.) in the drug sense.

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hang out (v.)

c. 1400, intransitive (as of the tongue, from the mouth); transitive use by 1560s; see hang (v.) + out (adv.). Colloquial meaning "to be found" is recorded from 1811, "in allusion to the custom of hanging out a sign or 'shingle' to indicate one's shop and business" [Century Dictionary]. As a noun (often hangout) "residence, lodging" attested from 1893; earlier "a feast" (1852, American English).

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knock up (v.)

1660s, "arouse by knocking at the door," from knock (v.) + up (adv.). However it is little used in this sense in American English, where the phrase means "get a woman pregnant" (1813), possibly ultimately from knock in a sense "to copulate with" (1590s; compare slang knocking-shop "brothel," 1860).

Knocked up in the United States, amongst females, the phrase is equivalent to being enciente, so that Englishmen often unconsciously commit themselves when amongst our Yankee cousins. [John Camden Hotten, "The Slang Dictionary," London, 1860]
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