Make sure the post is 'square' to the floor. [=make sure that there is a right angle where the post and the floor meet]
I'll go on the trip if I can 'square it with' my parents. [=if my parents will agree to let me go]
Here's the rest of the money. Now we're 'square'. [=now we don't owe each other anything]
(..) lowlifes and degenerates out on the street and you sell your little pussy for nothing... for some lowlife pimp who stands in a hole? I'm square? You're the one that's square, man. I don't screw and f$ck with a bunch of killers and junkies the way you do. You call that being hip? (..)1976 Taxi Driver
(..) You're a young girl. You should be home now. You should be dressed up, going out with boys, going to school. - You know, that kind of stuff. - God, are you square. Hey, I'm not square. You're the one that's square. You're full of shit, man. What are you talking about? (..)1976 Taxi Driver