Shat is the past tense of the verb 'to shit', which means to release solid waste from the body.
The word 'shat' is not polite and is usually used in informal or humorous situations.
Examples:
Yesterday, I shat in the toilet.
The dog shat on the carpet.
She shat in the woods.
The bird shat on my car!
I shat out a giant poop!
He shat in his pants and had to change.