HSN on AIM is otherwise known as "Having Sex Now". This definition came up after a friend recently got a girlfriend and would spend long hours "away". This acronym can be used as an Away message, or a subtle way to say someone is "busy".
Friend 1: Hey I will brb, gimme like one sec.
Away Message: aWaY... (aka HSN)
....One hour later....
Friend 2: Hello?
Friend 1: I am back.
Friend 2: Where did u go?
Friend 1: O it was jus my gf.