Unmentionable (adjective)

1. Not suitable or appropriate to be spoken or written about in polite or public conversation.
2. Taboo or embarrassing to mention.

Origin:

Derived from the verb 'mention' meaning to speak or write about something.

Examples:

1. The unmentionable subject was not discussed at the dinner table.
2. It was considered unmentionable to discuss sex in public.
3. The topic of death was once considered an unmentionable subject.
4. She kept the unmentionable secret locked away in her heart.
5. The unmentionable event was swept under the rug.