baby-sitter

BABY-SITTER offer a good example of what is called back-formation of a word. Usually nouns derive from verbs, but in the case of BABY-SITTER, this process is reversed. BABY-SITTER is an Americanism first recorded in 1937, while the verb to " baby-sit" was born from it over 10 years later. The noun has come to mean one who takes care of children or anything else that requires attention.
Source: QBP Encylopedia of Word and Phrase Origins, By Robert Hendrickson