Meaning

noun (sport) the mistake of occupying an illegal position on the playing field (in football, soccer, ice hockey, field hockey, etc.)

adjective illegally beyond a prescribed line or area or ahead of the ball or puck

adverb illegally in advance of the ball or puck

originFrom off- + side. Compare German Abseite (“remote area, backside”), German abseits (“out of the way, apart, aside”, adverb), Dutch afzijdig (“neutral, impartial”).

Etymology from Wiktionary, CC BY-SA 3.0