Shat is the past tense of Shit which is the present tense of crap, so, i shat my pants is saying i shit my pants, which means i crapped my pants, which means i took a poop in my pants, which means I dumped a lump
A Fat Shat is piece of excrement that has built up so much pressure in its time in the small intestine that, on release, reaches a speed of over 35MPH. Fat Shats can often cause damage to the toilet, usually cracking the porcelain. Fat Shats are usually as large as a small football. It is common for Fat Shats to be released in someone's pants when they are hit by a strong force, hearing shocking news, or possibly just when one cannot possibly hold it in any longer
My mom had a Fat Shat and now we have to go to Lowes to get a new toilet.
I shat myself is an even more extraordinary sentence for I shit my pants, you don’t want to go around saying “I shit my pants” but instead you could say “ I shat myself” you’ll be the coolest kid at school.