An exclamation that is a shorthand way of saying "we're gonna blow up" - say that phrase fast enough and it sounds like wiggle burp, and that is how wiggle burp originated.
Usage: during any multiplayer game, if a player is losing, he/she says 'wiggle burp!' This is generally used in games where you blow up when you lose, such as RTS games (where your base blows up), or games where you control some sort of tank or mechanical unit. It can also be used if a certain large/powerful unit is about to be destroyed, such as cruisers in Homeworld. In this case it is as if the crew of the ship are running around screaming "we're gonna blow up!".
However, usage of wiggle burp has developed beyond just these cases, and can be used in any case where loss is inevitable, even in racing games, sports games, etc.
(player is about to lose)
Player: "wiggle burp! wiggle burp! wiggle burp!"