1. [adj]  not appropriate for a purpose or occasion; "unsuitable attire for the office"; "said all the wrong things"
2. [adj]  not worthy of being chosen (especially as a spouse)
3. [adj]  not meant or adapted for a particular purpose; "a solvent unsuitable for use on wood surfaces"
4. [adj]  not conducive to good moral development; "the movie is unsuitable for children"
5. [adj]  not capable of being applied; "rules inapplicable to day students"

UNSUITABLE is a 10 letter word that starts with U.