A polite way of telling someone you haven't heard a word they said. Commonly used on long conference calls, when the speaker is monotonous, boring, or couldn't make a point of one were drawn for them.
Joe: "Blah, blah corporate office blah, blah, leverage proactively blah, blah human capital and grade-A synergy, blah, blah. Do you agree, Jim?"
Jim: "I'm sorry, Joe, I was multitasking, can you repeat that?"
The ability of some to change their facebook status, comment on someone else's status, like/dislike someone's status, add a photo, add to someone's wall, etc. on more than one electronic device at a time.
Dad: Where's your mom?
Tristan: She's on the computer, again. Changing her status or something.
Dad: I thought she was on her cell phone?
Tristan: She is...the computer, the cellular, her tablet. She's facebook multitasking.
A nice way to say that you're doing many different things at the same time. And since no one can divide by 0, that means you're doing many different things half-assed.
Joe thought that he would multitask by doing his physics homework during his math class. His multitasking, however, got him a detention and a failed homework grade instead.