A one-night stand between two individuals who are unknowingly siblings and/or cousins. May also lead to marriage and children. Stemming from the root 'Kentuckis' which means to breed within the family to enhance certain DNA traits. And Downpour refers to the lifestyle one can expect to have when birthing and raising a family of downsy children on minimum wage. It is not uncommon for each member of this type of family to be born with a creative number of limbs and vital organs.
"My cousins Bobbi Jo and Mary Lou accidentally engaged in a Kentucky Downpour the other night. Now they think they want to get married!"