Green’s Dictionary of Slang

a second brew of tea, the once-used leaves are left in the pot and a new kettleful of boiling water poured over them.

1889		G.R. Sims Horrible London 148: The lodgers divide their food frequently, and a man seeing a neighbour without anything will hand him his teapot, and say, ‘Here you are, mate; here’s a bull for you.’ A ‘bull’ is a teapot with the leaves left in for a second brew.
1909		J. Ware Passing Eng. of the Victorian Era.
1945		Baker Aus. Lang. 83: The English slang use of bull, for a teapot with the leaves left in for a second brew.