Green’s Dictionary of Slang

to defecate.

1976		A. Bleasdale It’s a Madhouse (1986) 110: Teddy wee-weed on the floor [...] But Teddy never poo-pooed.
1984		Partridge DSUE (8th edn) 908: [...] later C.20.
1992		D. Burke Street Talk 2 123: Do you need to go poo-poo?
2001		S. King Dreamcatcher 598: He has to go poopoo.