Etymology
Advertisement

offside (adj.)

also off-side, offsides, "on the wrong side;" from off (prep.) + side (n.). From 1867 in various sporting senses, originally in English football, between the ball and the opponent's goal during play.  

Advertisement
Advertisement
Definitions of offside
1
offside (n.)
(sport) the mistake of occupying an illegal position on the playing field (in football, soccer, ice hockey, field hockey, etc.);
2
offside (adv.)
illegally in advance of the ball or puck;
3
offside (adj.)
illegally beyond a prescribed line or area or ahead of the ball or puck;
the touchdown was nullified because the left tackle was offside
Synonyms: offsides
From wordnet.princeton.edu