used in chat/AIM slang as a way to denote that the other person has just received a minor slap to the back of the head, like what Leroy Jethro Gibbs gives his team.
The slap is a punishment, usually done when the person has done/said something stupid.
Can be used as an expression of aggravation
H0tChick101: I met this hot guy...
TalkaTalk20: His name?
H0tChick101: James. He's tall, auburn haired, and hazel eyed.
TalkaTalk20: <Gibbs>. That's my boyfriend.
DiNozzo: I've got this great new toy
McGee: Great, now we won't hear the end of it
DiNozzo: <Gibbs> Probie, come on