KOS
KOS has 2 meanings
Kid over shoulder
In chat and text messages, KOS often stands for "kid over shoulder." This acronym means a child might be reading your current conversation.
As such, you may want to keep the remainder of your conversation free of swears and other explicit language. You also shouldn't send any messages that you don't want your conversational partner's child to read ...
Example
KOS, just so you know!
S***, I'll stop swearing
Take the pop culture acronym challenge
In online gaming, KOS is a command to kill an enemy (or another player) on sight. This acronym is used in FPSes, MMORPGs, and other online multiplayer games.
For example, in an FPS, your teammates may send you KOS when they want you to shoot an enemy as soon as you see them. Or, in an MMORPG, your guild may apply the KOS status to another's members as part of a guild war.