Italian slang for a dangerous sex game in which the woman is the master and the man is the slave. The woman picks an object in the room and sticks it in the man's butt. Cérnita means selection in Italian language. In this case the selection is the one of the object picked by the woman. Be careful with Italian women saying "Ti faccio una cérnita", "I make you a selection".
Giovanna: "Ti faccio una cérnita stasera" - "I make you a cérnita tonight"
Marco: "No grazie, stasera preferisco andare al cinema con i miei amici" - "No thanks, tonight I'd rather go to the movies with my friends"
Pietro: "Ciao Marco, come stai?" - "Hi Marco, how are you doing?"
Marco: "Non molto bene. Ho difficoltà a camminare perchè Giovanna mi ha fatto una cérnita ieri sera" - "Not very well. I'm struggling to walk 'cause Giovanna made me a cérnita last night"