Green’s Dictionary of Slang

(Aus.) a crocodile.

1925		Teleg. (Brisbane) 21 Feb. 6/2: This is the first time that any of the local fishing enthusiasts have heard of a ‘scaly’ being caught on the line, but it is well known that Victoria Creek and other watercourses around are full of the monsters.
1943		Baker Popular Dict. Aus. Sl. (2nd edn).
1989		M. Walker How to Kiss a Crocodile 13: Will, our intrepid bare-handed croc catcher, hung continuously over the bow with bulging eye-balls scanning the shiny, moonlit lily pads for those telltale eyes of ‘old scaly’.