| ugly American | | | |
| ugly customer | | | |
| ugly duckling | | | |
| under a cloud | | | |
| under a spell | | | |
| under construction | | | |
| under control | | | |
| under erasure | | | |
| under glass | | | |
| under lock and key | | | |
| under no circumstances | | | |
| under one roof | | | |
| under one's hat | | | |
| under one's own steam | | | |
| under one's thumb | | | |
| under one's very eyes | | | |
| under one's wing | | | |
| under sail | | | |
| under someone's nose | | | |
| understand the assignment | | | |
| under the carpet | | | |
| under the covers | | | |
| under the gun | | | |
| under the hammer | | | |
| under the illusion | | | |
| under the impression | | | |
| under the influence | | | |
| under the mahogany | | | |
| under the microscope | | | |
| under the radar | | | |
| under the rose | | | |
| under the rug | | | |
| under the saya | | | |
| under the sheets | | | |
| under the sun | | | |
| under the surface | | | |
| under the weather | | | |
| under the wire | | | |
| under way | | | |
| undress with one's eyes | | | |
| unicorn barf | | | |
| unicorn puke | | | |
| unicorn vomit | | | |
| university of life | | | |
| university of the streets | | | |
| unknown quantity | | | |
| unload | | | |
| unmask one's batteries | | | |
| uno ab alto | | | |
| unplug | | | |
| unring a bell | | | |
| unsung hero | | | |
| until hell freezes over | | | |
| until one is blue in the face | | | |
| until the cows come home | | | |
| until the cows freeze over | | | |
| until the last dog is hung | | | |
| unto the ages of ages | | | |
| unused to | | | |
| up against | | | |
| up against it | | | |
| up a gum tree | | | |
| up a height | | | |
| up and at 'em | | | |
| up and away | | | |
| up-and-comer | | | |
| up and coming | | | |
| up-and-coming | | | |
| up and down | | | |
| up and running | | | |
| up a tree | | | |
| up for | | | |
| up for grabs | | | |
| up front | | | |
| up hill and down dale | | | |
| uphill battle | | | |
| up in the air | | | |
| up on | | | |
| up one's own ass | | | |
| up one's sleeve | | | |
| up on one's ear | | | |
| upon the instant | | | |
| upon the tapis | | | |
| upper crust | | | |
| upper-crust | | | |
| upper hand | | | |
| upper storey | | | |
| up poo creek | | | |
| ups and downs | | | |
| upset the applecart | | | |
| up shit creek without a paddle | | | |
| up shit's creek | | | |
| up shit's creek without a paddle | | | |
| up someone's street | | | |
| up the ante | | | |
| up the boohai | | | |
| up the creek | | | |
| up the river | | | |
| up the wall | | | |
| up the walls | | | |