Green’s Dictionary of Slang

(UK prost.) a poor woman's or street prostitute’s lover (and pimp).

1889-90		Barrère & Leland Dict. Sl., Jargon & Cant I 323/2: Dotty [...] An appellation used for one’s man by females of the lower classes or [sic] prostitution.
1890-1904		Farmer & Henley Sl. & Its Analogues II 312/2: Dotty, subs. (common).—The fancy man of prostitutes of the lowest type.