Not fit to be mentioned or discussed; unspeakable: unmentionable words. (adjective)
One that is not to be mentioned.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "unmentionable" in a sentence
"However, I am irrationally obsessed with the idea of mosquito bites in unmentionable places."
"To not a few, and not all of them readers of a certain unmentionable newspaper in Chicago, the constitutional monarchy seemed an expensive anachronism."
"Englishmen will welcome the new American playright with the name unmentionable to ears polite, and will recognise in him, as _the_ Dam _par excellence_, their brother, as one of the uncommon descendants of A-DAM."