The basic idea is that English articles make it difficult to tell if someone is saying “a nap” or “an app”

In older English, the word for the material that you wore while cooking was “napron”. After a few centuries of saying “a napron”, folks soon shifted to saying “an apron”.

The “n” moves the other way, too. When we can currently talk about “a nickname”, folks using Old English would talk about “an ekename”.

This is called rebracketing because [a] [napron] became [an] [apron] and someone was using brackets. It’s also called misdivision because the words were divided up wrong, maybe? Another term is metanalysis because they don’t want you to know what they’re talking about. I mean, really, how does a word like metanalysis tell you anything about what it means? But I digress…

After finding out about this and nadder and otch (yeah, that’s where notch came from) and even numpire, I’ve come to distrust any nouns starting with “n” or vowels.

I feel like a nidiot.

Notes:
I first heard about this on the History of English podcast ( http://historyofenglishpodcast.com/ ). To flesh out details, I relied on https://en.m.wikipedia.org/wiki/Rebracketing?wprov=sfti1 and  https://www.quickanddirtytips.com/education/grammar/how-a-napron-became-an-apron