Their are many definitions to pepecaca.
1- When u peepee and caca in the toilette
2-A game played in class similar to the penis game(when u yell the name louder and louder with one of your buddies)
Person 1: dude, I had the best fucking pepecaca last night
person2: n0ice.. was it a clean poopie?
Person1: nah man i fucking wish
Person2: yeah.. i fucking love clean poopies(when u dont have to whipe)
Person1: dude, u so pwned that 1.
Person: OMFG PEPE-FUCKING-CACA!!