Expression of delight or joy, often at unexpectedly good turn of events.
Written form often capitalised. Origin believed to be corruption/mistype of Yay. YSY is to "Yay" what teh is to "the".
YSY I won $500 playing on-line poker last night.
You single yet. When you know a really good looking girl who has a boyfriend and your in line for some fun.
Man: "Hey sexy, YSY?"
Female: "Nah, not yet"
Man: "Damn, can't wait to tap that"