What it means
A mild, slightly old-school British put-down for someone who's being daft, vacant, or having a proper little brain wobble. You'd call your mate a noddy when they've done something obviously silly, but with a grin rather than any real malice. It gives off harmless fool energy, a bit childish, a bit teasing, and it lands closer to muppet or numpty than a proper insult.
Usage examples
"He turned up to Tesco in his dressing gown, tried paying with an Oyster card, then looked offended when it bounced. Proper noddy."
"I was a right noddy, spent ten minutes looking for the glasses that were on my head the whole time."
"Stop being a noddy and read the label before you put the red sock in the white wash."
"You left your keys in the front door all night, you absolute noddy."
"Don't be a noddy, mate, the answer's literally on the screen."
Where it comes from
It comes from Noddy, the Enid Blyton kids' character. His name slid into British slang as a soft little jab for someone acting daft or a bit empty upstairs. That children's-book link is exactly why noddy feels teasing and harmless, not properly nasty.
