The word unsuitable is a relatively modern English formation, built upon the established adjective suitable. The core concept of "suit" originates from the Latin verb sequi, meaning "to follow," which evolved in Vulgar Latin and Old French to denote things that "follow" well together or are agreeable. The suffix -able was attached to create suitable, meaning "capable of suiting or being appropriate." Finally, the ancient Germanic negating prefix un- was prefixed to form its direct opposite. Thus, the word's logic is transparent: it literally describes something as "not capable of being appropriate."

Structure: un (not) + suit (appropriate) + able (capable of) - un: A prefix of Germanic origin (Old English un-), meaning "not." It is a common negating prefix in English. - suit: From the verb suit, meaning to be appropriate or convenient. It entered English via Old French (suite) from Latin sequi (to follow), with the sense of "be agreeable or fitting" developing in Romance languages. - able: A suffix from Old French -able, from Latin -abilis, meaning "capable of, fit for, or worthy of." It forms adjectives from verbs.