A forbidden or sacred thing set apart, from Tongan tabu, set apart, forbidden, the most famous Pacific loanword in English.
Documented
Cook's journal
Documented: the first English attestation is Cook's own journal of the third voyage, 1777, observing Tongan custom. The Tongans used tabu as an adjective; Cook's borrowing made it a noun for the prohibition itself and a verb, to taboo. Cognates span Polynesia, tapu in Maori and Tahitian, kapu in Hawaiian.
Assessment: Documented: A Voyage to the Pacific Ocean, 1784 edition, and the American Heritage Dictionary's etymology. One journal entry seeded a global word.
Stories
The missionary flattening
The documented drift: in Tonga, tabu marked what was sacred or reserved, for gods, chiefs, priests, as much as what was forbidden; English kept the forbidden and lost the holy. A word about consecration became a word about restriction, and then about workplace conversation topics.
Assessment: Stories: the reduction is the story of every sacred word that travels through a navy. The sacred does not survive steerage.
Interesting claims
The comprehensive meaning
Cook himself flagged it: very comprehensive meaning. Modern anthropology spent two centuries rebuilding what one Tongan dinner party demonstrated, that a single word can hold the sacred, the dangerous, and the prohibited in one concept, and that English needed to borrow the concept because it had split them apart.
Assessment: Interesting: the borrowing documents a gap in English, not just in English vocabulary.
Sources and further reading
Captain James Cook, journal of the third voyage, 1777, and A Voyage to the Pacific Ocean, 1784 (first English attestation)
American Heritage Dictionary etymology of taboo (Cook's borrowing; Tongan adjective to English noun and verb)
Polynesian cognates: Maori tapu, Hawaiian kapu (the documented family)
Worth asking
What does it cost a word to mean both sacred and forbidden at once?