Green’s Dictionary of Slang

a lodger who seduces his landlady, or the wife of his landlord; thus used of anyone who seduces a woman while her partner is absent.

1940s		in Campbell & Campbell War Paint 44: [aircraft nose art] Roger the Lodger.
1951		(con. 1941) E. Lambert Twenty Thousand Thieves 113: I’ve had some bad news [...] Roger the Lodger.