Simplify the following text for a Grade 6 native English-speaking person.
Original: `` Taboo '' : Common etymology traces taboo to the Tongan word tapu or the Fijian word tabu meaning `` under prohibition '' , `` not allowed '' , or `` forbidden '' .
Simplified: The English word is traced back to Tongan tapu or the Fijian tabu . These words usually mean `` not allowed '' , or `` forbidden '' .