This comes from The Newsroom S01E06, about 51:48.
Will comments on a large black bodyguard's advancements on McKenzie:
Will: Dress up like a tailback and he won't be able to lay a hand on you.
Bodyguard: Shrimp boat, if I wasn't being paid to keep you alive…
It's obviously received as a witty and offensive joke, but what is its meaning? What does tailback mean in this context?