A goalsneak is an

Australian rules football Australian football, also called Australian rules football or Aussie rules, or more simply football or footy, is a contact sport played between two teams of 18 players on an oval field, often a modified cricket ground. Points are scored by k ...

player whose job is to kick goals in a game, especially as a crumber. It usually describes a forward player, particularly a player in a forward pocket, who is small and nimble. A goalsneak may use his pace and cunning to snatch a goal after a running play where a larger player would typically use brute strength and/or height to mark the ball.

Sources

Australian rules football terminology {{AFL-stub