baby paps, n. — Green’s Dictionary of Slang

baby paps n.

also baby’s paps

[rhy. sl.; ? resemblance of a cap to the breast]

caps.

1857		‘Ducange Anglicus’ Vulgar Tongue.
1859		Hotten Dict. of Modern Sl. etc.
1859		Matsell Vocabulum.
1873		Sl. Dict.
1960		J. Franklyn Dict. of Rhy. Sl. 35/1: baby’s pap Cap, from c. 1855 [...] The term went into decline when caps ceased to be the exclusive fashion for workmen and charwomen – c. 1918.