Green’s Dictionary of Slang

to defecate.

1596		‘Misdiaboles’ Ulysses upon Ajax 30: The picture of a fellow in a square cap, scummering at a privy.
1598		Florio Worlde of Wordes n.p.: Chinchimurra... A skammering of a dog.
1640		R. Brathwaite Ar’t Asleepe, Husband? 83: [T]his loose Lecher could not inwardly bee more polluted, than his fayre Sattin Suite [...] was found hatefully scummered.
1656		‘On the Praise of Fat Men’ in Wardroper (1969) 214: This further know: fat folks do scummer / As much as cows do give in summer.
1672		J. Phillips Maronides (1678) VI 59: And plagu’d ’um with a Wiltshire Drummer / Till they were forc’d to scowr and scummer.
1673		‘Letter from a Missionary Bawd’ in Carpenter Verse in English from Tudor & Stuart Eng. (2003) 425: Lord Blessington presents the male Baboon, / Beating false time and scammering at the tune.