Why is the ninth month called September?
1550s, "one given to change," from change (n.) + diminutive suffix -ling. Meaning "person or thing left in place of one secretly taken" is from 1560s; specific reference to an infant or young child (usually stupid or ugly) supposedly left by the faeries in place of one they took is from 1580s. An earlier word for it was oaf or auf.
in European folklore, a deformed or imbecilic offspring of fairies or elves substituted by them surreptitiously for a human infant. According to legend, the abducted human children are given to the devil or used to strengthen fairy stock. The return of the original child may be effected by making the changeling laugh or by torturing it; this latter belief was responsible for numerous cases of actual child abuse.