Tuna sub backwards is "bus a nut". Men use backwards tuna sub when they're too ashamed to admit during sex that they're going to climax much quicker than they had previously thought.
Johnny: "Oh no! I'm about to backwards tuna sub!"
Jenny: "It's only been five minutes! Pull out now, you loser!"
Jenny: "It's only been five minutes! Pull out now, you loser!"
by ( ͡°╭͜ʖ╮͡° ) June 3, 2016