[under the thumb of] See: [UNDER ONE'S THUMB].

[under the weather] {adv. phr.} In bad health or low spirits. […]

[under the wire] {adv. phr.} With a narrow time limit; in […]

[under wraps] {adv.} or {adj. phr.} Not allowed to be seen […]

[understand] See: [GIVE ONE TO UNDERSTAND].

[underway] {adv. phr.} In progress; in motion. * /The yearly fund-raising […]

[Uninstalled] Euphemism for being fired. Also: decruitment.

[unknown quantity] {n.} Someone or something whose value and importance are […]

[unseen] See: [SIGHT UNSEEN].

[until all hours] {adv. phr.} Until very late at night. * […]