A Ninnyhammer is an extremely silly person, stupid and foolish.
A Ninnyhammer is prone to making gaffes and generally engaging in annoying behaviour.
The origin of Ninnyhammer is believed to be ninny from “Innocent” for someone who is naive, and “Hammer” as in hammered or stupid.
In the tird volume of J R R Tolkien’s The Lord of the Rings we find: “You’re nowt but a ninnyhammer, Sam Gamgee: that’s what the Gaffer said to me often enough, it being a word of his.”
“That bloke is a complete ninnyhammer, let’s vote to eject him from the club!”.