The way I heard it, and I cannot back this up as I wasn’t there… The origins of the phrase “tuna beigel” was from a certain yeshiva back during the Veitnam War. When the draft was in effect, many frum boys enrolled in yeshiva to avoid going to the US army. As such, many chasidish boys, to whom the yeshivish derech was a whole other world, found themselves sitting in bais medresh. Well, there was one table in the back where the chasidish boys would sit and everyday they came to yeshiva with, yes you guessed it… Tuna Bagel Sandwiches. This led to the group being called “Tuna Bagels” – meaning, those boys who do not fit in with their surroundings. Today, we see tuna baigels as “modern” chasidish boys, cool chasidish boys, or rebelious chasidish boys. the term all gives the same conotation to all these types. When a person looks one way but acts another, not fitting in with the surroundings or when the picture doesnt match the actions. Chasidish boys who grew up sheltered, but now have every high tech gadet, cherry lights, and cell phone.
I hope this clears things up.