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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?
It is a gibe at the bodyguard's past as a football player:
Lonny was a promising Defensive Tackle for Clemson University, SC, and was drafted into the NFL but cut 5 weeks into the season. - The Newsroom Wiki
A tailback in American football is a fast and elusive running back who receives handoffs and shallow passes and advances the ball by running with it. One of a defensive tackle's primary jobs is to tackle running backs before they can advance the ball; Will implies that Lonny was a failure in the National Football League because he couldn't lay a hand on his targets.
