umbrage, n. [resentment].
umpire, n. [referee], [judge], [arbitrator], arbiter.
unabated, a. [undiminished], unassuaged.
unable, a. [incapable].
unabundant, a. [scarce], [deficient], [rare].
unacceptable, a. [objectionable], unwelcome, [undesirable].
unaccountable, a. [incomprehensible], inexplicable, [inscrutable].
unaccustomed, a. [unfamiliar], unwonted, [unused].
unacquainted, a. [unfamiliar], [strange].
unadulterated, a. [unsophisticated], unalloyed, [pure].