Un- means not, none

uncertain (adj) - not sure

uncommon (adj) - rare; not the usual; remarkable

unconscious (adj) - not within thought; not awake

undecided (adj) - not yet at the point of making a decision

unexpected (adj) - not what someone thought would happen; not predicted

unmistakable (adj) - clear; cannot be understood the wrong way; not able to be confused or misunderstood

unnecessary (adj) - not required

untidy (adj) - messy; not neat; not organized

unwise (adj) - not smart

unworthy (adj) - not deserving; not having any value