The term off-side derives from the military term "off the strength of his side". When a soldier is "off the strength", he is no longer entitled to any pay, rations or privileges. He cannot again receive these unless, and until he is placed back "on the strength of his unit" by someone other than himself.

In football, if a player is off-side, he is said to be "out of play" and thereby not entitled to play the ball, nor prevent the opponent from playing the ball, nor interfere with play. He has no privileges and cannot place himself "on-side". He can only regain his privileges by the action of another player, or if the ball goes out of play.