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Wiktionary
knocking shop

alt. (context chiefly British slang English) brothel. n. (context chiefly British slang English) brothel.

Wikipedia
Knocking shop

Usage examples of "knocking shop".

He puffs away at his cigar, its tip flaring red in the gloaming of the knocking shop.

Basically, it was a knocking shop for people who liked having sex with aliens.