It's an onomatopeic snigger.
An expression of mirth.
(
Crude But Effective A weapon of mouse destruction,
Thu 13 Jul 2006, 15:22,
archived)
I though it was off-a-ma-topic
Ba-dum-tish*
(*the sound of people hitting me)
(
Chexworth will one day make Yoko Matsugane his bride...,
Thu 13 Jul 2006, 15:29,
archived)