fabruncle Share definition Flag When a man and his sister have sex and conceive a male child. the man is the childs fabruncle. It is his father , brother and uncle. Cletus is my fabruncle. He had sex with my mom his sister)She is my sister and my aunt also. fabruncle by Jim shues January 23, 2018 0 0 Get merch Get the fabruncle mug. Buy Now — $32.95 See more merch