IM-pounce 

To initiate conversation, the second a person logs into their IM.

X)waiting for sign in
Y)signs in
X)IM-POUNCE

IM-pounce by Silent0809 February 19, 2009

IMpounce 

When someone jumps on you immediately when you sign in to messenger or another instant messaging application.

I *just* wanted to check my emails and go to bed when Steve IMpounced me and I had to chat for another half hour!

IMpounce by jbng October 23, 2007