free lunch
/ˌfriː ˈlʌn(t)ʃ/noun
1Originally and chiefly US. A lunch provided free of charge in a bar, saloon, etc., as a means of attracting customers. Frequently attributive Now historical.
2 Figurative. 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.
Origin
Mid 19th century; earliest use found in New York Herald. From free + lunch.