Oaf \ohf\ , noun;
1. A clumsy, stupid person; lout
2. A simpleton; dunce; blockhead
3. Archaic: A deformed or mentally deficient child; a changeling
Oaf is first attested in 1638 and is a variant of auf ("a changeling; a foolish child left behind by the fairies"), which derives from Old Norse alfr ("elf, fairy").
In case you didn't know (I didn't), a changeling is "a child surreptitiously or unintentionally substituted for another" or "an ugly, stupid, or strange child left by fairies in place of a pretty, charming child."