by 1974 in sense of "devotee of the rock music band the Grateful Dead;" earlier (with lower-case) "one who rides for free on the railroads" (1866), and "non-paying spectator" (1841).
Entries linking to deadhead
Grateful Dead
San Francisco rock band, 1965, the name taken, according to founder Jerry Garcia, from a dictionary entry he saw about the folk tale motif of a wanderer who gives his last penny to pay for a corpse's burial, then is magically aided by the spirit of the dead person. A different version of the concept is found in the Egyptian Book of the Dead.