Used to indicate that the speaker is totally unimpressed, unexcited and/or unsurprised by some item of news or gossip (s)he has just received. It is an adaptation of the phrase woo hoo, which is used to indicate excitement, and was popularized by British rock band Blur in their hit single Song 2.
"Hey, guess what? Janet Jackson flashed her boob on the Superbowl half-time show!"

"That saggy-titted old witch? Woo fucking hoo!"
by Mooseboy February 11, 2004