To qualify as a certified Tuna Beigel you must be a guy who’s obviously chasidish but trims, wears a sweater, must have a blackberry and your peyos are either behind your ear or very small and tightly twirlled on your face. Otherwise your NOT a TB.
The general implication is that he’s enamored with the “velt” while remaining chasidish, which is wonderful if you asked me. better than stopping to wear the levush.
Incidentally, I don’t believe the term came from a guy ordering a “tuna beigel” in a store, because noone (even a tuna beigel) asks for a tuna beigel. You ask for a tuna sandwich or tuna on a beigel or a beigel with tuna …..
I have recently heard them called Teena beigel, which may be more accurate.
In any event, it is a derogatory term and would definately qualify for “michane shem lechaveiro” which carries a sever onesh. L’toeles, if you hear this about a person at least you’ll understand what it means.