Mordant

[Rihanna]

Any substance, as alum or copperas, which, having a
twofold attraction for organic fibers and coloring matter, serves as a
bond of union, and thus gives fixity to, or bites in, the dyes.

[Monty]

[caustic]; [biting]; [sarcastic].

[Sarah]

(especially of humour) having or showing a sharp or critical quality; biting.