It meant "un-cool" or not current with popular fashion and behavior.
Jerk IS a slang term. It means a person who doesn't care about anyone and is rude or mean to everyone. I can't find any specific slang from 1950 that means the same thing.
A square is a person who is not "cool" or "hip." This would be the person who is unpopular and does not follow the latest fashions in clothing, slang, and behavior. "Square" is an old slang term from the 1960's.
"Fresh" or "minty" would be 1950's slang terms for new.
The term "baloney" emerged in the 1950s as slang for nonsense or foolishness. It likely derives from the idea that bologna sausage (or baloney) is made up of various mixed meats, hence something that is a mixture of different things and therefore nonsensical.
fruit
There are 88.2579 square meters in 1950 square feet.
Platters.
The gang.
If you mean slang as in internet slang, then it means "Certified Old Fart".
Saturday Square - 1950 was released on: USA: 7 January 1950
It means you're old-fashion and you always follow the rules. Pretty much you won't take any risks and your "square" is your comfort zone.
"Don't sweat it" was a slang expression in the 1950s that meant "not to worry."