Sphincter Giggle

The act of shitting yourself while laughing at something. This happens because you were laughing so hard, your anus laughed with you.

Friend 1: 'My little sister told me the Justin Bieber movie was the best experience of her life and started to cry'

Friend 2: 'What did you say?'

Friend 1: 'I started laughing uncontrollably, but then I had a Sphincter Giggle and ran to the bathroom to clean up'

Friend 2: 'OMG I think I just had one too!'