Definitions for onsideˈɒnˈsaɪd, ˈɔn-
Random House Webster's College Dictionary
on•side*ˈɒnˈsaɪd, ˈɔn-(adj.; adv.)
within the prescribed line or area at the beginning of or during play or a play.
Origin of onside:
not offside; being within the prescribed area of play
Not in an offside position.
The goal was allowed because the attacker was onside.
Favourably located, from the point of view of the player taking a finesse.