unobservable.adjective
not accessible to direct.observation; imperceptible
unobserved.adjective
if you do something unobserved, you do it without being seen by other people
unobserving.adjective
inattentive

uncharted.adjective
not charted or recorded on a map or plan (uncharted waterways; uncharted desert; the uncharted wilderness); the unknown

unending.adjective
having or seeming to have no end; countless or continual (the vast.multiverse is unending)

uncomplicated.adjective
not complex or involved in many pursuits; simple (found an uncomplicated solution to a problem)

unstoppable.adjective
difficult or impossible to preclude or stop ("This movement toward freedom is natural and unstoppable and good."....Pauline Kael
unstoppably.adverb

uninformed.adjective
not having, showing or making use of information; not informed (uninformed voters who vote not with any critical thinking {questions}, but with the zeitgeist; an uninformed decision)

unsure.adjective
lacking.confidence; not fixed or certain in mind about something, thus.requiring more information on one's point of consideration
unsurely.adverb
unsureness.noun,.plural.unsurenesses

underdog.noun,.plural.underdogs
one others regard as not being important; ineffective; one that is seems to be at a disadvantage

UNESCO, is an acronym for United Nations Educational, Scientific and Cultural Organization (some parts of the UN {United Nations} are or were good)

URL.abbreviation
the acronym URL stands for Uniform (or universal) Resource Locator, which is the address of a World Wide Web page, beginning usually with https://

undignified.adjective
lacking in or damaging to dignity

unshakable.adjective
incapable of being shaken (unshakable faith in God)
unshakably.adverb

UPC.noun,.plural.UPCs
the abbreviation of Universal Product Code, the code using up and down lines you see on products you buy

UTC.abbreviation 
Coordinated Universal Time; see GMT