A person who logs onto AIM or comes back from Away, fires off a message to a friend (usually important/personal) and then immediately logs off or goes Away without waiting for a reply.
<Jess2218 is online at 2:36:54 PM>
Jess2218(2:36:55 PM): Hey, what's up? OMG have you heard about Ashley? Want to go out tonight?
<Jess2218 is offline at 2:36:59 PM>
Charlotte54 (2:37:00 PM): Wait, what? Hello?
Charlotte54 (2:37:02 PM): Fuck, I've been hit by an AIMsniper!