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1.
/noun/
A planned attack, committed by two or more internet-savvy people, which uses instant message to assault, interrogate, or incriminate a third party.
My attempt to date two boys at once fell apart when they IMbushed me, demanding I fess up.
by Maureen O'Connor July 10, 2004
 
2.
To instant message a friend, co-worker or family member immediately after they become available online. The term applies most aptly to those who do so consistently and perpetuate lengthy and pointless instant messaging sessions.
Dick: "Did you get your fourth-quarter financials finished?"
Jane: "No, because my college roommate IMbushed me as soon as I booted up and I couldn't get rid of her"
by Jake Valentine December 20, 2007
 
3.
an IM ambush; the surprise attack of a person messaging you immediately after you sign on to a messaging service viz., AIM, Facebook Chat.
I become very wary when I sign on to facebook because I'm scared of an IMbush; they are very surpising and often awkward.
by yournamehere192 October 23, 2009