If something - particularly an idea or a suggestion - is described as 'off the wall', it is shocking or unusual. The origin of this expression comes from sports like handball, racquetball or squash, in which the ball is hit against a wall. When the ball comes 'off the wall', the player is uncertain where it will go. Therefore, an 'off the wall' idea is a rather unusual idea, and no one is entirely sure where it will lead. In other words, the outcome is unpredictable!