ninnyhammer
… it’s far from certain from the dates when they were first used whether ninny comes from ninnyhammer or the other way around. [...] Try the weekly World Wide Words e-magazine - it features words in the news, weird words, new(ish) words, old words, words people ask questions about, and even the occasional grovelling correction. [...] The first part, ninny, looks like the word we still know today, which is thought to come from a shortened and modified version of an innocent, because innocent at this time could mean a person lacking in intelligence or sense, who was silly, half-witted, or imbecilic. [...] Contact the author if you want to reproduce this piece, but first see my advice page, which also has notes about linking. [1]
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Your comments and corrections are welcome. [...] The second part is less obvious, but might be from hammer-headed, which could also then refer to a person who was dull-witted or stupid. [1]
The answers are right on your browser and just a click away with Dictionary.com Toolbar. [2]
This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional. [3]
For the most part it has vanished except in works that consciously seek to evoke a bygone age through antique language. [1]
Sources:
[1] World Wide Words: Ninnyhammer
[2] ninnyhammer definition | Dictionary.com
[3] Ninnyhammer - definition of Ninnyhammer by the Free Online Dictionary …