[two-time] {v.}, {slang} To go out with a second boy or […]

[TY] "Thank you"

[u] "You"

[UA] User agreement

[UFIA] "Unsolicited finger in the anus"

[ul] Unlucky (gaming term)

[UL, U/L] "Upload"; sending data to a remote host.

[Umfriend] One with whom one has a sexual relationship; as in: […]

[Under Mouse Arrest] Getting busted for violating an online service's rule […]

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