It just occurred to me that "ninnyhammer" doesn't make sense. A "ninny" a case of cerebral insufficiency. The "ninnyhammer" ought to refer to the exasperated boss
who has to try to get some work out of him.

It's in Webster's 1913 Dictionary, so it's been around at least that long. Maybe hammer was added kind of from another language; my husband, of German descent, would sometimes call our babies slobber-hanuses when they drooled. Or maybe it started off as ninny-yammering? That would make more sense than ninnies hammering...

And what's wrong with Anu's explanation?

I'll hammer you, you ninny!

I clearly recall that Samwise (who we learned means "half-wise" or maybe half-witted) Gamgee was speaking to himself, saying "You're nowt but a ninnyhammer, Sam Gamgee." I never had any doubt that the term meant "fool". I suspect there is a deeper derivation to it than Anu mentions. If the "ham" means "town" like I suspect, it probably is a derogatory derived from a placename.

Cheers,
Bryan

You are only wretched and unworthy if you choose to be.

Bryan! Oh my goodness, I thought you'd given us up for good! Welcome back, Sweet Thing! :-))

Brian sez:
If the "ham" means "town" like I suspect, it probably is a derogatory derived from a placename.

Hmmm... does the term "village idiot" come to mind?