When you're having anal sex with a girl which she finds too painful and her anus clenches tightly on you penis pinching it like a lobster claw.
"Dude, did you get anal off your girl last night?"
"Yeah but I went too deep and her brown lobster squeezed on me so tight it hurts when I pee!"
A brown lobster is a person who has spent so much time at the beach that they're skin has become brown and leathery.
"There's alot of brown lobsters at the beach today".
"You spend so much time at the beach, your becoming a brown lobster".