Unmentionable rhymes with:
unconscionable, actionable, fashionable, fissionable, impressionable, objectionable, unconscionable, unfashionable, abominable, actionable, determinable, fashionable, fissionable, imaginable, impressionable, inalienable, interminable, objectionable, personable, questionable, reasonable, seasonable, terminable, unconscionable, unfashionable, unquestionable
Unmentionable sounds like:
unattainable, undeniable, undeniably, untenable