Green’s Dictionary of Slang

sunny south n.

the mouth.

1887		Referee 7 Nov. in DSUE (1984).
1891		G.R. Sims ‘Tottie’ Dagonet Ditties 126: She’d a Grecian ‘I suppose,’ / And of ‘Hampstead Heath’ two rows / In her ‘sunny south’ that glistened / Like two pretty strings of pearls.
1935		Mirror (Perth) 14 Sept. 17/2: Mouth: Sunny south.