The tuna sandwich, not confused with its popular edible counterpart, is a term used with in gang societies, most notably used within Italian-American lore. When one asks if you would like a tuna sandwich it can mean two things, to congratulate a fellow man of a heralded feat, or more commonly a death wish towards whoever it is said. Only applicable if said in an Italian-American accent, if said plainly, it is just a request of literally eating a tuna sandwich.
Other versions of a tuna sandwich involve tuna melts, patty melts, or any combination of the two, such as a tuna patty melt. But one should be aware that at any time, if you're offered a "tuna patty melt" it is automatically a negative, life-threatening phrase.
"Hey Tony good job on bein made, you want a fuckin' Tuna Sandwich?"
"Hey you fuck! Johnny sends this tuna sandwich with his regards."
"Hey I like you kid, maybe come over and fuck my sister, and have a tuna sandwich."
"You want a tuna sandwich? You want a fuckin Tuna melt? You want a fuckin tuna patty melt?"