Today it means: In pre-modern European folklore: an infant of a magical creature that was secretly exchanged for a human infant. In British, Irish and Scandinavian mythology the exchanged infants were thought to be those of fairies, sprites or trolls; in other places, they were ascribed to demons, devils, or witches. Full definition →
The noun is derived from change + -ling (suffix with the sense ‘immature; small’). Sense 6 (“idiot, simpleton”) is from the idea that foolish children had been left by magical creatures (sense 1). The adjective is derived from the noun.
Etymology from Wiktionary, CC BY-SA 4.0.