Nothing, or something equivalent to nothing. Jack shit has the remarkable property that its absence and presence are identical. Typically used with or without a negative to describe a total lack of knowledge, value, or significance. Its use carries a strong negative connotation which can express frustration, disdain, ignorance, or other negative qualities.

It’s an acronym standing for Can’t Remember A Fucking Thing. It’s usually used to describe embarrassment at being unable to remember someone (CRAFT moment); going onstage as the lead in the latest theatrical performance and your mind goes completely blank (CRAFT situation); the next morning after a night when you got pissed as a fart and woke up in bed cuddling a traffic cone and a string of fairy lights wound round your prick.

Jack is the only son of Awe Schitt and O. Schitt. Awe Schitt, the fertilizer magnate, married O. Schitt, the owner of
Knee-deep Schitt, Inc.
Jack Schitt married Noe Schitt and they had 6 children: Holie Schitt, The twins; Deep Schitt and Dip Schitt, Fulla Schitt, Giva Schitt and Bull Schitt.
Jack and Noe divorced. Noe later married Mr. Sherlock and because her kids were living with them, she wanted to keep her previous name. She was known as Noe Schitt-Sherlock.
Dip Schitt married Loda Schitt and they had Chicken Schitt. Fulla Schitt and Giva Schitt married the Happens
brothers in a dual ceremony.
The Schitt-Happens children are Dawg, Byrd and Horse. Bull Schitt left home to tour the world. He recently returned with his new bride, Pisa Schitt.

Now, when someone say's you don't know Jack Schitt, you can correct them.

A term used to describe a lack of anything.

An American gentleman, with whom certain people are unfamiliar.

nothing,ziltch,zero,empty,contains absolutely nothing.