I never ever heard of this term. I would have thought, had I not read the reast of the posts, that a “tuna bagel” kind of a guy, is a chilled out guy, who is not into fancy restaurants or clothes, and is satisfied with the minimum of anything. He is happy with a tuna bagel, doesn’t need the Barney’s suits or steak dinners, and doesn’t want that type of wife who is materialistic. Just goes to show you…
Calling someone a fruitcake is VERY uncomplimentary – means he is a nut job!