a submissive, feminine male's orgasm achieved exclusively through anal stimulation, no hands, oral, nothing. This can be done with toys, (dildos, strap-ons, etc.) but most commonly involves a bigger, more dominant man brutally pounding the G-spot in the sissy's ass until they cum hands-free
After getting absolutely pounded by his hook-up for an hour, Alex had a sissygasm so large, it made his eyes roll into the back of his head. Now, he's not sure if he can ever go back
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...