"Shit myself" is when you do bad at something such as a game of Cock of Duty or a game of HALER 45. You just know you fucking suck and can't help but say it.
Boy 1: YOU FUCKING SUUUUCK! *Kills other person*
Boy 2: Awwwh, I just shit myself.
1. A mexican pastime
2. A popular computer game series
When Mexicans get bored, it's time for grand theft auto.