An extremelyoffensive term used to degrade some one other than yourself. Similar to "you're cool," You're sweet is intended to irritate and provoke the recipient with obvious sarcasm.
Bob: Hey Chuck, sorry but I just drank all of your beer.
What a girl tells you when you do something nice for her. You think that she's into you so you ask her "the question." Only for her to tell you that you're like a brother to her. So you kys.
The speaker “half doomed” and the other person “semi-sweet” insinuates that although separately they’re only halves of a whole, together they complete each other.