Fly 13

Fly 13 (Verb): To engage in any; whether subtle or outright, act or display of incest.

The 13 refers to the number of stars on the Confederate (CSA) flag, which in recent years has been memed as a symbol of incestuous deeds, as well as a number of other redneck shenanigans that typically take place in America. So, to "fly 13" (Fly the flag with the thirteen stars) would mean, as a modern metaphor, to support or proudly take part in incest (Sexual acts or feelings taking place among siblings or close relatives).