Green’s Dictionary of Slang

brother-in-law v.

(US) to pursue clandestinely the wife or girlfriend of another man.

[US]D. Hammett ‘The Golden Horseshoe’ in Continental Op 64: ‘Where’s Kewpie?’ I asked. ‘Brother-in-lawing Ed?’ a big Swede girl leered at me. ‘I’ll see if I can find her.’.