Game: The act of killing an enemy that was weakened by another member of your team. This action will often upset your teammate and should be done with caution. Can also be used as a noun. (kser: kill stealer)
In-depth Version: It's 2 vs 2, you and your teammate and 2 enemies. Both of you have a specific person to kill.
Say your enemy is weakened and you only need more hit on him before you can gain exp/gold/etc.
Your teammate's turn is before yours, and on his turn he kills YOUR enemy. Congradulations! You've just been ksed.