Green’s Dictionary of Slang

king (death) n.

(bad) breath.

1908		C. Rook London Side-Lights 278: He had followed [...] for a couple of miles of hard running. ‘Most lost me king,’ he said to the cabman. [...] this was an instance of the rhyming slang, that ‘breath’ was once rhymed with ‘King Death’.
1932		‘P.P.’ Rhy. Sl. 5: ’Strewth, yer ‘King Death’ don’t arf ‘Pen and Ink’.
1938		J. Curtis They Drive by Night 64: He kissed her. Blimey, what a king death. Didn’t half pen.
1960		J. Franklyn Dict. of Rhy. Sl.
2002		D. Shaw ‘Dead Beard’ at www.asstr.org 🌐 Dionne’s king death is nice and sweet and she’s certainly kept her shape.