[1] : 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

[2] : WordNet (r) 2.0

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" syn: offsides ant: onside
     adv : illegally in advance of the ball or puck
see also:
offsides onside 

Offside >> O Words
Offside, definition of term: Offside
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