1560s, "sudden and apparently causeless turn of mind," of unknown origin. Perhaps it is from a dialectal survival of a word related to Middle English friken "to move nimbly or briskly," from Old English frician "to dance" [OED, Barnhart]. There is a freking attested in mid-15c., apparently meaning "capricious behavior, whims." Or perhaps from Middle English frek "eager, zealous, bold, brave, fierce" (see freak (n.2)).
The sense of "capricious notion" (1560s) and that of "unusual thing, fancy" (1784) preceded that of "abnormally developed individual or production" (first attested in freak of nature, 1839, which later was popular in variety show advertisements for bearded ladies, albinos, etc.; compare Latin lusus naturæ, which was used in English from 1660s). As "drug user" (usually appended to the name of the drug) it attested by 1945. The sense in health freak, ecology freak, etc. is attested from 1908 (originally Kodak freak "a camera buff"). Freak show is attested from 1887.
also freakout "bad psychedelic drug trip," or something comparable to one, 1966, from verbal phrase freak out, attested from 1965 in the drug sense (from 1902 in a sense "change, distort, come out of alignment"); see freak (n.). There is a coincidental appearance of the phrase in "Fanny Hill:"
She had had her freak out, and had pretty plentifully drowned her curiosity in a glut of pleasure .... [Cleland, "Memoirs of a Woman of Pleasure," 1749]
where the sense is "she had concluded her prank."
1580s, "act of keeping under authority and regulation, fact of checking and directing action," from control (v.). Meaning "a check, restraint" is from 1590s. Meaning "a standard of comparison in scientific experiments" is by 1857, probably from German Controleversuche. Airport control tower is from 1920; control-room is from 1897. Control freak "person who feels an obsessive need to have command of any situation" is by 1969.
1972 (also as a verb), originally in phone phreak, one of a set of technically creative people who electronically hacked or defrauded telephone companies of the day.
The phreaks first appeared on the US scene in the early 1960s, when a group of MIT students were found to have conducted a late night dialling experiment on the Defense Department's secret network. They were rewarded with jobs when they explained their system to Bell investigators. ... The name "phone phreak" identified the enthisiasts with the common underground usage of freak as someone who was cool and used drugs. [New Scientist, Dec. 13, 1973]
The ph- in phone may have suggested the alteration, and this seems to be the original of the 1990s slang fad for substituting ph- for f- (as in phat).
"sideshow freak," 1916, U.S. carnival and circus slang, perhaps a variant of geck "a fool, dupe, simpleton" (1510s), apparently from Dutch gek or Low German geck, from an imitative verb found in North Sea Germanic and Scandinavian meaning "to croak, cackle," and also "to mock, cheat" (Dutch gekken, German gecken, Danish gjække, Swedish gäcka). The modern form and the popular use with reference to circus sideshow "wild men" is from 1946, in William Lindsay Gresham's novel "Nightmare Alley" (made into a film in 1947 starring Tyrone Power).
"An ordinary geek doesn't actually eat snakes, just bites off chunks of 'em, chicken heads and rats." [Arthur H. Lewis, "Carnival," 1970]
By c. 1983, used in teenager slang in reference to peers who lacked social graces but were obsessed with new technology and computers (such as the Anthony Michael Hall character in 1984's "Sixteen Candles").
geek out vi. To temporarily enter techno-nerd mode while in a non-hackish context, for example at parties held near computer equipment. [Eric S. Raymond, "The New Hacker's Dictionary," 1996]