taboo (adj.)

also tabu, 1777 (in Cook's "A Voyage to the Pacific Ocean"), "consecrated, inviolable, forbidden, unclean or cursed; prohibited to a certain class," explained in some English sources as being from Tongan (Polynesian language of the island of Tonga) ta-bu "sacred," from ta "mark" + bu "especially." But this may be folk etymology, as linguists in the Pacific have reconstructed an irreducable Proto-Polynesian *tapu, from Proto-Oceanic *tabu "sacred, forbidden" (compare Hawaiian kapu "taboo, prohibition, sacred, holy, consecrated;" Tahitian tapu "restriction, sacred, devoted; an oath;" Maori tapu "be under ritual restriction, prohibited").

The noun ("prohibitory restraining injunction") and verb ("to put under taboo") are English innovations first recorded in Cook's account [OED, 2nd ed., 1989].