careless talk, n. — Green’s Dictionary of Slang

careless talk n.

[rhy. sl.; ult. a ref. to the WWII posters with the legend ‘Careless talk costs lives’]

a stick of chalk, usu. as used by darts players for scoring.

1957		Eve. News (London) 31 Oct. cited in Franklyn (1960).
1960		J. Franklyn Dict. of Rhy. Sl.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1992		R. Puxley Cockney Rabbit.