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]
(..) the old lady's liquor cabinet. Yeah, well, you shouldn't drink. - Why not? - Because you... You might regret it later in life. Marty, don't be such a square. Jeez, you smoke too? You're beginning to sound just like my mother. Marty? Why are you so nervous? Lorraine... (..)1985 Back to the Future