1US historical A lunch provided free of charge in a bar, saloon, etc., as a means of attracting customers. Frequently attributive.
2figurative Something that is (seemingly) free of charge or cost; frequently in phrases implying that everything inevitably involves a cost of some kind, as "there is (also ain't) no such thing as a free lunch".
Mid 19th century; earliest use found in New York Herald. From free + lunch.