The term jit is originally prison slang. It just means somebody younger than you. However, it is most often used to refer to young thugs/wannabe thugs or other punky kids. For instance, you wouldn't call your 10-year-old cousin a jit.

a young muthafucka that don't know shit

Another word for Kid, youngsta

A young person, usually between the ages of 12 and 15, that thinks they are alot older/cooler than they actually are. Often can be found at the mall or movie theater on a friday night in groups of 5-10.

How to spot a jit...
girls- seen wearing too much makeup, clothes that do not fit correctly, and a fake designer purse
boys- often a foot or so shorter than the girl, seen wearing the most "ganster" clothes possible even though they are often rich young white kids

Miami and southern Florida slang for a "young gangster".

Younger kids (usually middle skoolers) who think theyre ghetto, hood, skater, stoner, and/or surfer. They use the old and killed joke, Your Mom, even when it makes zero sense. Boy jits usually wear overly baggy jeans, have buzzcuts or moppy long hair, havent or barely have hit puberty and are shorter then the skanks they hang out with. They try to say "bro", "dude", or "man" in every sentence. Girl jits usually try to act ghetto and or hood and like total sluts when they are skanky. They listen to Lil Wayne, Akon, etc. They wear ridiculously short shorts and other small clothing and Nike Airs.

Juvenile In Trouble. From the corrections system, often used by one kid in detention refering to another, usually younger deliquent.