There are two ways to define the terms SIDEARM in volleyball.
Is a serve which is lie an overhand serve but it was a serve before and overhand serve was invented. You basically Take the ball and put it in your throwing hand for an overhand serve. Then turn your whole body sideways. Then hit the ball out of your hand that the ball is currently in.
another sidearm to some people is when you go and dive or hit a ball with one arm. It can be called a sidearm but it is mostly called a one-arm hit.