The Collaborative International Dictionary of English v.0.48:

offside \offside\ offsides \offsides\adj. (Sports)
   Illegally beyond a prescribed line or area or ahead of the
   ball or puck; -- in sports such as football or hockey; as,
   the touchdown was nullified because the left tackle was
   offside.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

offsides
    adj 1: illegally beyond a prescribed line or area or ahead of
           the ball or puck; "the touchdown was nullified because
           the left tackle was offside" [syn: offside, offsides]
           [ant: onside]