jack all - A person who knows a little about a lot of things, but can’t

A person who knows a little about a lot of things, but can’t master one. They’re like a hot mess with a PhD.

"He knows about music, math, and pizza." "Yeah, he’s a jack all."

"He knows about cars, cats, and karate." "Yeah, he’s a jack all."

"He knows about basketball, beer, and being annoying." "Jack all, for sure."