The term is a textual mutation of the Cantonese slang "18-C (Sap Baat C)" or the English numeric slang for adult entertainment districts. Specifically, "18" refers to the legal age of majority, while "1 Night" refers to the "one-night stand" culture or overnight entertainment packages common in Chinese KTV (Karaoke Television) bars.
There is a semi-mythical documentary produced by or Vice Media (in its early Canadian days) titled "One Night in Guangzhou" or "18 & Life: China's Sex Trade." In 2004, a reporter named Adrian F (often confused with a Hong Kong actor) went undercover. 18 1 night in china 2004 uncensored english