taboo

English borrowed "taboo" from Tongan tapu, meaning "prohibited, sacred," first recorded around 1777. The word traces back further through Proto-Polynesian *tapu, Proto-Oceanic *tabu, and Proto-Eastern Malayo-Polynesian *tambu.

The shift from tapu to taboo happened simply because English speakers heard the Tongan p as a b.

Taboo is a doublet of "kapu," meaning the two English words descend from the same ancestral root but arrived by different paths.