Green’s Dictionary of Slang

1. a son.

1960		J. Franklyn Dict. of Rhy. Sl. 37/1: bath bun Son. 20 C. Cockney. A poor and pointless specimen, hence seldom used.
1969		S.T. Kendall Up the Frog.
1980		Barltrop & Wolveridge Muvver Tongue 64: ‘Come back with your hair on, you bald-headed bathbun.’ However, a lot of Cockney kids who sang that were of Irish descent. ‘Bathbun’ was a way of not-quite saying ‘bastard’.
1992		R. Puxley Cockney Rabbit.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.

2. the sun.

1971		‘Jack Jones’ Rhyming Cockney Sl.
1992		R. Puxley Cockney Rabbit.
2002		M. Coles More Bible in Cockney 27: The lemon is coming when I will make the Bath bun go down in the middle of the blinkin’ day.