Green’s Dictionary of Slang

latchpan n.

[SE latchpan, a dripping pan, lit. a ‘catching pan’]

the lower lip; thus hang one’s latchpan, to sulk, to pout.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1864, 1867, 1870		Hotten Sl. Dict.
1935		A.J. Pollock Und. Speaks.