> Not many people are aware that Berk is a piece of Cockney rhyming slang,

I'd say at least the ~70M population of the uk would know it. Probably quite a few outside also

Midwest United States we used to use the word Berk interchangeably with times we would use the word asshole.

I never really understood why, but now that I understand it's rhyming slang I'm even more confused as to why we did that.

Which part of the midwest?

Middle of Iowa. Why?

Interesting, I wonder how localized it is?

As for why, because I've never heard the term and am also from the area.

Ah. I assume it was super local. We used to travel to Nebraska and/or Illinois quite a bit to see family, and they never said it.

Interesting! I enjoy local vernacular. But the way I read your original message was more like declaring a midwest generalized usage, which may not have been your intent.

In my childhood in New Zealand I remember berk used to mean plonker/fool.

Wiktionary says:

  berk (UK, Ireland, Commonwealth) a fool, a prat, a twit, etc. Although the term remains in fairly wide use, its specific origin and meaning in rhyming slang is less well known, lessening its vulgarity. Synonyms see fool and idiot.

In the AU / NZ domain the Burke and Wills expedition of 1860-61 helped, fairly or unfairly, cement the association of 'Burke' with foolish.

https://en.wikipedia.org/wiki/Burke_and_Wills_expedition

I've never once heard the word "berk" used in the UK.

Not the case at all. I'd say, of those people who are familiar with the word, fewer than 5% are aware of its origins.

Moss used it in IT Crowd. That’s where I heard it first, and then looked up the origin.