An Invisibob is a special avatar glitch from Gaia Online. This avatar glitch was only possible on April Fools' Day of 2004.
Since the YGS, or "You got Served" boxes, exploded in your avatar's face, it looked like there was soot on it.
The Administrators had planned for it to go away after 24 hours, but it didn't - they'd stay until you saved changes to your avatar. Thus, this led to two types of rare avatars: the Sootfaces and Invisibobs.
An Invisibob was created by deleting your avatar data, then clicking on a YGS box. The resulting explosion glitched the avatar system and forced your avatar to go to the default base - a Soot-face and a pair of floating boxers.
...See one on Gaia Online, by doing a search in the memberlist for "Sevi Rais.".
"Sevi Rais", "Weeping Willow", and "Teh Ghost of Bap" are all Invisibobs.
I wish I were an Invisibob. Instead I'm an Invisibob Wannabe.