The act of uncontrollably laughing, or having a giggle fit, at something that could actually be funny, or possibly not. A haver of a sillygasm will convulse while laughing, grab on to things tightly, and can not be easily stopped, as if they were having an orgasmic experience.
Guy: The joke wasn't that funny, but she just started having a sillygasm, I think she was drunk...
When one is overwhelmed by the amount of Silly-Bandz one has or the amount a store has in stock!
Mallory got a SILLYGASM because Michael showed her all his 392 Silly-Bandz.