When you send somebody a message over AOL Instant Messenger (or other IM service), and it causes them to send half of what they were in the middle of typing to you in a message. Also used to describe when the "your buddy is typing" indicator comes up immediately upon sending a message, meaning that the text typed in the IM box is not really for you.
12:20:01 SomeGuy123: Hey. what's up?
12:20:01 ThatChick234: nd.
12:20:10 SomeGuy123: HA! Aim Sniped...
12:20:16 ThatChick234: very funny.