It of course could be 2 pieces of sesame seed bun with some cooked lamb in the middle like a sandwich and known as a lamb burger- However the far more common use for this term is a descriptive reference to insult those who attempt/ or are known to have sexual intercourse with sheep or lambs. Commonly conversationally in Derby in the Uk and the whole of Wales.
"I really can't believe what I read about that guy with the sheep in the field that's disgusting"

"Hey man I could have told you, he's been smashing those lamb burgers for some time Baaaaaaa!"

"Dude, too much.. That's sick and not in a good way"
by Vegetablerightsandpeas July 28, 2016
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