Saying that you're a sandwich is the equivalent to saying that you're a submissive in a BDSM relationship. It can also mean that you have to desire to he in the previously mentioned relationship in which you are the submissive
Jonah: I'm a sandwich
Oliver: What does that mean?
Jonah: It means I want my girlfriend to peg me
by I'm_A_Sandwich January 6, 2022
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From the blues classic - slang for sexual availability.

"Free tonight?"
"I'm your tuna fish sandwich."
"I'm your tuna fish sandwich, spoon me on your bread.
I'm your tuna sandwich baby, best to eat in bed."
by Monkey's Dad February 20, 2023
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