A very difficult maneuver requiring the participant to go through the motions of eating a girl out but with your chin. This term is most commonly referred to as chinning or chin. The crimson chin is when one chins a girl who is on her period causing the blood to be stained on the chin, leaving a crimson hue on the chin, dubbing the crimson chin.
1. v. to eat a girl out while she is on her period.
alternatively,
2. v. to throat-fuck an individualto the point that your balls slap against their face with such voracity that their chin becomes chapped and red, thus giving them a "crimson chin".
Daringly, these two actions can be performed at the same time to result in what can only be described as potentially one of the leakiest sex acts.