Act of one person, usually a female, sitting on another person's face to sexually stimulate that person. Can include cunnilingus, breath control. See femdom.
That old bugger has a facesitting fetish.
Or "facesitting". When the person (usually a female on a male, or female on a female), sits on her sex partners face. The person being sat on will either have their nose in their partner's pussy or their nose in their asshole, or their mouth in their partner's pussy or asshole.
She performed a facesit on him because he requested to smell her asshole.