When you are chatting with several people at once via IM (instant messenger).
"No, I haven't been able to look at that website you just told me about. I'm multichatting."
Simultaneously having an online IM chat with several people while also sitting within conversation distance of said people in real life, especially on Facebook.
Random guy: Hey, what are you guys all laughing about?
Chatting guy: We're all multichatting on Facebook.
(gets a message)
Chatting guy: LOL, he totally does!
Chatting more than one person at once
dude 1 : ayo lets chat
dude 2 : sorry dude i am multichatting.
chatting online with multiple chattees at the same time
I was multichatting this morning with 12 people at the same time, a personal best, but hard to control.