A marriage between first-cousins. The marriage is considered a "horseshoe" marriage due to the marriage's connection in a marriage resembling a horseshoe on a family tree (the daughter of one side connects to the parents of the son of his parents as the parents have blood-sibling relations.
"Hey Dave, did you hear that Bryan's marrying Rachel?"

"Yeah man, it's a horseshoe marriage though so they might have to get married in another state."

"Damn man, incest..."
by Sir Cosby Sweater February 25, 2014
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