An oaf is a clumsy, slow, or foolish person, almost always a man when people use the word. It is the sort who knocks the table and then says the wrong thing. It is an insult, though sometimes a soft one, the way you might call a friend a big oaf when they have been clumsy, not cruel. It is informal, and a little old; you would not put it in a school report. Stories often picture an oaf as large, but the word is about the clumsy, blunt manner, not about size as a fact. Though actually, it probably comes from Old Norse álfr, “elf”. In old belief a dull or clumsy child might be explained as a changeling left by elves. So oaf and elf are, surprisingly, from the same root. Most people who say oaf are not thinking of elves at all.