Old English hær "hair, a hair," from Proto-Germanic *hēran (source also of Old Saxon, Old Norse, Old High German har, Old Frisian her, Dutch and German haar "hair"), perhaps from PIE *ghers- "to stand out, to bristle, rise to a point" (source also of Lithuanian šerys "bristle;" see horror).
Spelling influenced by Old Norse har and Old English haire "haircloth," from Old French haire, from Frankish *harja or some other Germanic source (see above). Hair-dye is from 1803. To let one's hair down "become familiar" is first recorded 1850. Homeopathic phrase hair of the dog (that bit you), remedy from the same thing that caused the malady, especially a drink on the morning after a debauch, 1540s in English, is in Pliny.
1795; Figurative use by 1841. Hair perhaps in reference to the slight pressure required to activate it.
The difference between a hair-trigger and a common trigger is this—the hair-trigger, when set, lets off the cock by the slightest touch, whereas the common trigger requires a considerable degree of force, and consequently is longer in its operation. [Charles James, "Military Dictionary," London, 1802]
also longhair, 1893, "cat with long hair," from long (adj.) + hair (n.). As "intellectual," especially in musical tastes, "devotee of classical music," 1920 (late 19c. long hair was noted as a characteristic of classical musicians, perhaps inspired by the famous locks of Liszt). Sense of "hippie" attested from 1969. The adjective long-haired is attested from mid-15c.
Forty years ago, a music teacher who was not born abroad and who did not wear long hair was regarded with suspicion. He was spurious—not the real thing. On the face of it he could not be a good musician. [W. Francis Gates, in "The Music Student," vol. i, no. 3, October 1915]
