castor, adj. — Green’s Dictionary of Slang

castor adj.

[? SE castor sugar; thus parallel to sweet adj.¹ (2); Simes, A Dict. of Australian Underworld Slang (1993), prefers the old criminal signal of tugging one’s hat or castor n., to indicate ‘all clear’, used fig. in the non-criminal world]

(Aus.) excellent, admirable, first-rate; thus be on the castor with, to be popular with.

1943		Sydney Morn. Herald 11 Dec. 7/2: When the plans have progressed without a hitch, the situation is described succinctly as ‘she’s castor,’ or ‘she’s sweet’.
1953		K. Tennant Joyful Condemned 214: We’ve got an eighteen-foot stick [...] and a ladder made of strips of flour bags. She’s castor.
1955		Argus (Melbourne) 2 July 41/2: She’s castor, mate, your credit’s grouse.
2003		McGill Reed Dict. of N.Z. Sl. 44: castor Approval of anything excellent, from castor sugar. Early C20 ANZ.