1.Lit. to move out of a line of people. If you step out of line, you will lose your place in it. I had to step out of line to sit down on the curb and rest for a minute.
2.Fig. to misbehave; to deviate from normal, expected, or demanded behavior. If you step out of line again, I'll slap you. Tom stepped out of line once too often and got yelled at.