In football, ball-carriers run towards defenders who are attempting to tackle them. By positioning the ball securely in one arm, the ball-carrier can fully extend his other arm, locking his elbow, and outstretching his palm. Then, the ball-carrier pushes directly outwards with the palm of his hand onto the chest or shoulder of the would-be tackler. The fend is a pushing action, rather than a striking action.
A stiff-arm fend may cause the tackler to fall to the ground, taking him out of the play. Even if the tackler keeps his feet, it becomes impossible for him to complete a tackle, as he cannot come close enough to wrap his arms around the ball-carrier.
A well-executed stiff-arm fend can be a very powerful offensive weapon. It can often completely chane the direction of play.
The term don't argue was coined in Australia to describe the stiff-arm fend. The term describes what a commentator imagined the ball-carrier might be saying as he shoved his opponent in the face or chest, and is used as a noun.