[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 […]

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

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

[until hell freezes over] {adv. phr.}, {slang} Forever, for an eternity. […]

[up a tree] {adv.} or {adj. phr.} 1. Hunted or chased […]

[up and about] or [around] {adv. phr.} Recovered and able to […]

[up and at them] 1. {adv. phr.} Actively engaged in a […]

[up in the air] {adj.} or {adv. phr.} 1. {informal} In […]