changeling

noun

1. a child surreptitiously or unintentionally substituted for another.
2. (in folklore) an ugly, stupid, or strange child left by fairies in place of a pretty, charming child.
3. Philately. a postage stamp that, by accident or intention, has been chemically changed in color.
4. Archaic.
   1. a renegade or turncoat.
   2. an imbecile.

noun

1. a child believed to have been exchanged by fairies for the parents’ true child
2. archaic
   1. an idiot
   2. a fickle or changeable person