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.
Tunasub was created by some filthy minds on the twitterverse as an allusion to masturbation. Backwards it spells bus a nut. Used as an incognito way to announce that one has or has the intentions of masturbating.
Yo man, me and my girl were having sex and right before I busted my nut she yelled "TUNASUB" then started ticking me. I swear man, she loves tunasubbing me more than she loves food.