- U/L
- Upload, send to the BBS
- UAE
- Unrecoverable Application Error (Bane of Windows users!); Unix Amiga Emulator; United Arab Emirates
- UBD
- User Brain Damage
- UL
- Urban Legend; UpLoad, send to BBS
- UNCLE
- Unified Nuclear Coalition of Latvian Emigrants
- UOK
- yoU OK?
- URLCM
- yoU aRe weLCoMe
- USU
- USUally (frequently just "usu.")
- UTC
- Under The Counter; Coordinated Universal Time (French initials)
- UTT
- Under The Table
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