U is For ...
Umbrella - When I was little, I loved a book about Davey and the Big Yellow Umbrella. The umbrella was so large that Davey's father cut square holes in it, inserted clear  plastic, and taped up the sides, so Davey could see through.
Unix - I was introduced to the Unix operating system shortly after meeting Richard (who had recently purchased a Sun Microsystems workstation). Unix is my preferred  operating system for programming. I've been working with Unix since 1983.
Untersuchungen - One of the words I learned in my College German class (Reading German for Scientific Knowledge). The English translation is "investigations".
Upside Down - Our cats love to lie upside down, exposing their fluffy pettable tummies. If they do this a lot, can the position really be termed "upside down? What. after all, is the "up" side of a cat whose frequent position is tummy up?
U is also for Useful, Umbra, Ukelele, Ubiquitous, Upbeat, Uvula, Unicorn, Ulterior, Ultimate, Ultraviolet, Umlaut, Umpteen, Unanimous, and Un-.