ninnyhammer
A foolish person; a simpleton.
Why, the more fool she; aye, the more ninny hammer she.
1608, A Yorkshire Tragedy, dubiously claimed to be by William Shakespeare
Where it comes from
Apparently from ninny (although attested slightly later) + -hammer, of unknown origin; perhaps compare earlier hammer-headed.
Adapted from the Wiktionary entry ninnyhammer (CC BY-SA 4.0).