adjectiveinformal, rareAustralian, New Zealand
Honest, above board; true. Also: fair, just. specifically Sport (of a game, match, etc.): fairly, honestly, or gamely contested; (also of a participant) playing, fighting, etc., with spirit, courage, and integrity.
Late 19th century (in an earlier sense). Either (i) from square + dinkum, after fair dinkum.