Mear pa'rriba

Mear pa'rriba literally means "to urinate aiming up." In Mexican slang, this expression means "to behave in a way that is harmful to oneself." In other words, when you meas pa'rriba you ruin your immediate future with your own actions. The parabola is a powerful parable. 

Example:

José: Acabo de ir a recursos humanos a proponer una estructura más horizontal para que aumente la participación en la toma de decisiones y se limiten el poder y los abusos del jefe. Quedé muy satisfecho.
Feliciano: Nunca había visto a un soberano pendejo tan contentote después de haber meado pa'rriba. ¡Pinche animal!

Jose: I just came back from HR. I proposed a more horizontal structure to increase the participation in decision making, and to put limits on the boss' power and abuses. I'm pretty happy about it.
Feliciano: I never saw such an oblivious dunce, so happy after having harakiri-ed his career. ¡Poor moron!

Papita, flan

Papita literally means "small potato", and flan is a delicious type of custard deeply loved by Mexicans.  When properly prepared, both potatoes and flan are tender and easy to eat voraciously, without even needing to chew. When something is said to be papita or flan, that something does not pose a challenge. It is "a piece of cake".

Example:

Felipe: Al chile, ser un verdadero líder no es de enchílame otra. Es un reto súper grueso.
Vicente: Bájale. Mi señora me dice que está bien papita.  

Felipe: Straight up, being a true leader is not something easy. It is an overwhelming challenge.
Vicente: Bring it down. My lady said it's a piece of cake. 

Of course, my horse

There is no other expression that depicts the playfulness of Chilango speak better than this one. "Of course, my horse" means "by all means, buddy". It is not offensive but it is very informal. 

Mexicans take English courses all their academic life (K-12). Among the handful of expressions and words that stick is "of course". Since no Mexican would consider fun such a dry expression, a rhyming companion is added: "my horse". It doesn't imply that the listener is a farm animal that needs to be ridden on the back. Not at all. It just rhymes. "Of course, my horse" is equivalent to "mos def".

Example:

Poncio: ¿Ya calaste al Instituto Mexicano del Sonido?
Pilatos: Of course, my horse. Esos broders si se rifan.

Poncio: Have you listened to the Mexican Institute of Sound?

Pilatos: Mos def. Those dudes are awesome.

No dar paso sin huarache

Huarache (wa • DUH • che) is a Mexican word for a rudimentary sandal. Sandals (the most primitive form of shoes) protect our feet every step we take. No dar paso sin huarache literally means "not taking a step without preparation or without being aware and  ready for the consequences."

Example:

Gastón: Sabía que tus pinches invitados se iban a acabar las chelas en dos patadas. Así que me tomé la libertad de poner la lavadora a  buen uso, con un guardadito pa' los cuates.
Aurelio: Estás cabrón. Tú sí no das paso sin huarache.

Gaston: I knew your damned friends were gonna go through the beer in no time. Thus I took the liberty of put the washing machine to a good use, hiding some reserve for the homeys.
Aurelio: You're too much. You're always prepared for what lies ahead.  

Licar

Licar (lee • CAR) is an alternative way to say "to take a look". It is  not offensive. However, its use its rather informal and conveys that the speaker knows the ways of the street. Use it in combination with other rather rough terms in order to be consistent in the image you project with your speech.

Example:

Copiloto: ¿Licaste a la reinita en primera clase que está súper sobres? Me late que no es nada apretada.
Capitán: Neta, párale al cotorreo y ponte al tiro que ya vamos a despegar.

Copilot: Did you check out the hottie in first class that looks very interested? It seems she's game.
Captain: Straight up, drop it and get yourself prepared. We're about to take off. 

Chicle y pega

Chicle y pega (CHEE • klei • PEH • gah) literally means "chewing gum and sticks". It is used to imply that a strategy might work but it is not guaranteed. The idea is that such strategy "might be like chewing gum and stick."

Example:

Rutila: Compré una serie completa de la lotería. Si ganamos, vamos a ser millonarios.
Nicanor: Chicle y pega y no tendríamos que chambear de nuez en toda nuestra vida.

Rutila: I bought a lottery ticket. If we win, we'll be millionaires.
Nicanor: If that happens, we won't have to work again for the rest of our lives.  

The árbol del chicle (chewing gum tree) in Coyoacán is a popular attraction for tourists (who take pictures of it) and locals (who get rid of their chicles on it)

Ya salió el peine

Ya salió el peine literally means "the comb already came out". It is used to convey that finally some crucial information about a matter has been revealed. A mystery has been solved or the culprit's identity has been disclosed. 

Example:

Enrique: ¡Chale! ¿No te parece difícil de entender que el achichincle del gobernador lo haya balconeado de que tenía una amante?
Marcelo: Nel, pus si ya salió el peine. El achichincle estaba bien clavado con la amante.

Enrique: Geeez! Don't you find hard to understand that the governor's aide made public the relationship of the governor and his lover?

Marcelo: Nope, it's all clear now. The aide was head-over-heels in love with the guv's lover.