The story
Unknown. Earliest recorded use in the phrase fair dinkum in 1879, from Australia. Also recorded (slightly later) from Derbyshire and Lincolnshire dialect. Perhaps related to Gloucestershire dialect ding (“to work hard”).
First known use, by meaning
- From 1894 Genuine, true, honest, on the level.
- From 1882 Hard work.
Etymology from Wiktionary, CC BY-SA 4.0.