Green’s Dictionary of Slang

(W.I.) a male lover; esp. one who spends money on his girlfriend.

1913

H. De Lisser Jane’s Career (1971) 101: The innocent query drew forth a scream of laughter [...] ‘Miss Burrell want to know what a “back force” is!’ and then they exploded anew. [...] Then one girl condescended to explain. ‘It is a “friend“,’ she said.