Oaf (ōf), n. [See Auf.] 1. Originally, an elf's child; a changeling left by fairies or goblins; hence, a deformed or foolish child; a simpleton; an idiot.
[1913 Webster]

2. A clumsy or awkward person.
Syn. -- klutz, clod, lummox, stumblebum.
[PJC]