[YIM] Yahoo! Messenger

[YM] "Your mom!"

[YMMV] "Your mileage may vary" or "you make me vomit"

[YMSTICPC] "You Might Say That, I Couldn't Possibly Comment"

[YNK] "You never know"

[YOMANK] "You owe me a new keyboard" (after a joke causes […]

[yon] See: [HITHER AND THITHER] or [HITHER AND YON].

[yoo-hoo] {interj.} - Used as an informal call or shout to […]

[you bet] or [you bet your boots] or [you bet your […]

[you can say that again] See: [YOU SAID IT].