Peeing on the inside is laughing by yourself about something very funny without showing that you are laughing. ; or where you are somewhere you can't laugh loud so you have to hold in your laughter.
*priest walks by and trips*
person 1; *turns to friend* oh my god, i'm peeing on the inside!!
*person listens in on another persons joke.*
person telling joke: how did the guy with no arms and no legs cross the highway? take the f out of free and the f out of way.
person 2: there's no F in way.
person 1: HHAHAH THERES NO EFFIN WAY.
eavesdropper texts friend "oh my god i'm peeing on the inside."