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 eyesroll 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...