-verb; a series of actions in which a male or female (generally a male), when confronted with the sibling of a former sexual partner, proceeds to engage said sibling in sexual relations, thus "doubling down"
-noun; a reference to the afore-mentioned aspect of copulating with one sibling and then the other at a later date, spoken of as though it were a status, achievement, or standard to be gained
(verb) "When Shawna's sister Latrice started dry-humping me at the costume party, I realized I had the chance to double down
without either one of them figuring it out."
(noun) "One of my lifetime goals is to achieve a Double Down
with identical twin blondes, giving one a Tony Danza
and the other a Bareback Surprise
. All together, I will have achieved the trifecta of sex maneuvers, thus sealing my name among the gods forever."