This is another really stupid Cassidy suggestion. He claims that the word nincompoop comes from the Irish naioidhean ar chuma búb, which he says is pronounced neeyan [er] um boob and (again, according to him) means “a baby in the shape of a blubbering boob.”
Let’s examine this claim carefully. Firstly, does the ‘Irish’ phrase sound like nincompoop? Not much. Is there any evidence of anyone ever using this expression before Cassidy? No, of course there isn’t. Is it likely that anyone would use it? Think about it. Insults need to be clever or punchy. They need to be effective as ways of putting someone in their place, which is why you very rarely find people saying things like “He is a man who seems to have the appearance of a dolt.” If you want to insult someone, you simply call them a dolt. Because of this, the chances of Cassidy’s claim being correct are vanishingly slight.
Although we don’t have any solid evidence about the real origin of nincompoop, there is nothing to suggest that it comes from Irish. It is first found in English in the 1670s. Some dictionaries conjecture that it probably comes from the phrase non compos mentis (a legal formula meaning not of sound mind). Others dispute this. But Cassidy’s ridiculous suggestion is just another confirmation that he was far from compos mentis himself.