The term was used by the sailors who manned German submarines during the war. As they arrived back in port after a cruise, they would prominently display one red triangular flag, called a "victory pennant," for each ship that they sunk.
Synonymous with a woman being "on-the-rag."
2. My girlfriend gets really horny when she's flying the victory pennant. It was good that we were at her place, because her bed linens were terribly stained by our fucking.
3. She told me that she her period had ended, but when I finished and pulled out, it looked like there had been a stabbing. I guess she was still flying the victory pennant.