"crack cocaine addict," slang, by 1986, from crack (n.) in the drug slang sense + head (n.). In earlier slang, crack-headed meant "crazy" (1796), from the literal sense of crack.
Entries linking to crackhead
crack n.
"a split, an opening, narrow fracture," mid-15c., earlier "a splitting sound; a fart; the sound of a trumpet" (late 14c.), probably from crack (v.).
Meaning "sharp, resounding blow" is from 1836. Meaning "rock cocaine" is first attested 1985. The superstition that it is bad luck to step on sidewalk cracks has been traced to c. 1890. Meaning "try, attempt" first attested 1830, nautical, probably a hunting metaphor, from slang sense of "fire a gun."
At their head, apart from the rest, was a black bull, who appeared to be their leader; he came roaring along, his tail straight an [sic] end, and at times tossing up the earth with his horns. I never felt such a desire to have a crack at any thing in all my life. He drew nigh the place where I was standing; I raised my beautiful Betsey to my shoulder, took deliberate aim, blazed away, and he roared, and suddenly stopped. ["A Narrative of the Life of David Crockett, Written by Himself," Philadelphia, 1834]
Adjectival meaning "top-notch, superior, excellent, first rate" (as in a crack shot) is slang from 1793, perhaps from earlier verbal sense of "do any thing with quickness or smartness" [Johnson], or from the verb in the sense of "speak boastingly" and suggesting "having qualities to be proud of" [Century Dictionary]. Grose (1796) has "THE CRACK, or ALL THE CRACK. The fashionable theme, the go." To fall or slip through the cracks figuratively, "escape notice," is by 1975. Crack-brained "demented" is attested from 1630s. The biblical crack of doom is in reference to the sound (Old English translates it as swegdynna maest).
head n.
Old English heafod "top of the body," also "upper end of a slope," also "chief person, leader, ruler; capital city," from Proto-Germanic *haubid (source also of Old Saxon hobid, Old Norse hofuð, Old Frisian haved, Middle Dutch hovet, Dutch hoofd, Old High German houbit, German Haupt, Gothic haubiþ "head"), from PIE root *kaput- "head."
Modern spelling is early 15c., representing what was then a long vowel (as in heat) and remained after pronunciation shifted. Of rounded tops of plants from late 14c. Meaning "origin of a river" is mid-14c. Meaning "obverse of a coin" (the side with the portrait) is from 1680s; meaning "foam on a mug of beer" is first attested 1540s; meaning "toilet" is from 1748, based on location of crew toilet in the bow (or head) of a ship.
Synechdochic use for "person" (as in head count) is first attested late 13c.; of cattle, etc., in this sense from 1510s. As a height measure of persons, from c. 1300. Meaning "drug addict" (usually in a compound with the preferred drug as the first element) is from 1911.
To be over (one's) head "beyond one's comprehension" is by 1620s. To give head "perform fellatio" is from 1950s. Phrase heads will roll "people will be punished" (1930) translates Adolf Hitler. Head case "eccentric or insane person" is from 1966. Head game "mental manipulation" attested by 1972.