the word "unmentionable" is a modern english formation built from classical components. the core, "mention," derives from latin mentionem, meaning "a calling to mind, a speaking of," from the verb minisci, "to think." the prefix "un-," of germanic origin, is a prolific negator in english. the suffix "-able," from latin -abilis, indicates a capacity or quality. thus, the literal construction "not capable of being spoken of" directly and logically yields the modern sense of something considered too taboo, improper, or embarrassing to name.

Structure: un (not) + mention (speak of) + able (capable of) un- (old english, a prefix meaning "not"); mention (latin, a root meaning "to speak of, name"); -able (latin, a suffix forming adjectives meaning "capable of, fit for").