Pelársela

Pelársela is a sharp and highly inappropriate expression, typically employed in one of two forms: pelársela a alguien (lit. to peel it to someone) and just pelársela (to peel it to oneself). The word comes from the action of pulling back the prepuce when an uncircumcised male is about to urinate (circumcised males do not possess a prepuce anymore). It's an allegory. If you can make someone do that for you it means that you are the unchallenged "boss" to that person. In the second form, the reflexive mode "me la pelé", it means that you recognize you are a loser, either because you let go a great opportunity, because you didn't meet some standard or beacuse of sheer bad luck. Its phallic reference is nowadays an etymological curiosity only.

Nadal se la volvió a pelar a Federer en el Abierto de los Estados Unidos => Federer showed Nadal who's the boss once more in the U.S. Open
Me la pelé con las vacaciones de diciembre => I won't have a Christmas break, I'm such a loser

A popular way to express to someone that you think you're better is telling him or her "tú, a mí, me la pelas". If at the same time you move up and down your right or left hand (both only if you already master the one hand version) as if you were about to roll the dice in a board game, the effect is exponenciated. Any Mexican will get your point right away and probably won't let you walk away without proving it. Beware.