Green’s Dictionary of Slang

rotter n.2

[in sceptical eyes a talker of rot n.1 (1)]

(Aus.) an expert.

[Aus]Baker Popular Dict. Aus. Sl. (2nd edn).
[Aus]N. Pulliam I Travelled a Lonely Land (1957) 26: ‘How do you mean — “rotter?”’ I asked. ‘Well, expert — dead hand — however you’d say it.’.