Real Dictionary

taboo
countable noun: If there is a taboo on a subject or activity, it is a social custom to avoid doing that activity or talking about that subject, because people find them embarrassing or offensive.

taboo in American English
among some Polynesian peoples, a sacred prohibition put upon certain people, things, or acts which makes them untouchable, unmentionable, etc.
sacred and prohibited by taboo
to put under taboo
adjective: proscribed by society as improper or unacceptable
noun: a prohibition or interdiction of anything; exclusion from use or practice
transitive verb: to put under a taboo; prohibit or forbid

taboo in British English
adjective: forbidden or disapproved of; placed under a social prohibition or ban
noun: any prohibition resulting from social or other conventions
verb: to place under a taboo