One of the mysteries of the English language finally explained.
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.
square dinkum/ˌskwɛː ˈdɪŋkəm/
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