tuna beigel?
I'm actually stumped on this one. But I'll give it a lame try
Ppl that grew up speaking Yiddish only, have a very peculiar accent once they do start speaking English. The accent is very apparent when these people say the word tuna bagel (pronounced: teena baaygel (kinda))
The term somehow came to be used to describe these chassidim when they are clearly trying to Americanize themselves and usually failing.
If you see a former chassid stereotypically wearing a tucked in Polo Shirt and Yankees hat with his peyos (side-locks) tucked away out of sight and a very trimmed "cool" beard he will very often be called a tuna beigel
Sorry if this was long and confused you even more I but it was the best I could do
I was not trying to insult anyone with this post (I hope I have not)