long-hair (n.)

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]