Instead of laughing, a "sniss" is a slightly stronger than normal exhale through your nose followed by a grin after seeing or hearing something slightly funny
An involuntary sneeze erupting just as one is to commence pissing thus producing a painful expansion of the urethra followed by a two-three days of dick hurt.
The simplest, most likely explanation for two chicks who spend all their time together is lesbianism. A variation on Occam’s Razor with respect for the scissoring art form.
"Tammy and Kristy bought Birkenstocks and went to Tulum again; they say it's a spiritual connection"
"Occam's Scissor, they're banging"