square up

You could hear it in Morcheeba's song, Aqualung:

It don't come cheap

Square up to fight
Like dynamite

So, to square up simply means to get ready to fight, get in the fighting position.

You could see also something like that:

US and China square up for a trade war.

Accidentally, to square up also has a different meaning, which is to settle a bill or debt, but this is a different story.