Cagado

Literally, cagado (kah • GAH • though) means "shat", "shitted", "shit", or whatever past participle you choose for the verb "to shit". In Mexican slang something cagado if it is very funny. It is an informal expression but it is not offensive, and has absolutely nothing to do with feces. Use it confidently.

The video of the little boy dancing quebradita (a dance characterized by small, frequent jumps) is a good example of something cagado.    

Oclayo

Oclayo (awe • KLAH • jaw) means eye, and it is a derivative of ojo (lit. eye). It is typically used in conjuntion with echar, as in echar un oclayo (lit. throw an eye), which means "to keep an eye" or "to take a look." 

Example 1:

Porfas, échale un oclayo mientras voy a echar la firma.
Would you please keep an eye on this while I go to take a leak? 

Example 2: 

Le eché un oclayo a unas boclas de segundo cachete muy chidas, me latieron mucho.
I saw a pair of second-hand speakers pretty cool, I really liked them.

Spanish filmmaker Luis Buñuel shocked the world in 1929 with his surrealist short movie "Un Chien Anadalou" (An Andalousian Dog). The film opens with a scene in which the oclayo of a woman is cut with a razor blade. Échale un oclayo a la película.

Ni paper-mate

Ni paper-mate (nee • pay • per • mate) is an expression that would make you sound like a proper Chilango slang speaker. It is a derivative of ni pedo (lit. "not even fart") which is an interjection to convey frustration and solidarity, similar to "what can you do about it!" when actually nothing can be done. Paper-mate is a brand of pens and retractable pencils.

Example:

Demóstenes: Iba en mi democrático, me cuajé y me bajaron el billuyo de la quincena.
Cucho: Ni paper-mate. Al monte pío.

Demóstenes: I was riding the bus, fell asleep and my pay of two-weeks was stolen.
Cucho: What can you do about it! Now you gotta go to the pawn shop.

Paper-mate ad, 1980s.

No canta mal las rancheras

Rancheras (lit. "from the ranch") are songs from the countryside highly popular before Mexico became a mostly urban country. They are often played by a mariachi band and follow a waltz-ish rhythm. The expression no canta mal las rancheras means "(s)he is not bad at singing ranch-style music". This colorful expression is used to convey that, against what some could expect, a person has an above-par performance or has above average attributes.

Example: 

Gualberto: Eres un culero. Pusiste a tus empleados a parir chayotes, haciéndolos chambear en días feriados.
Benito: Pues tú no cantas mal las rancheras, maestro. Tus empleados se quejan de que te vale verga cuando te piden días de incapacidad.   

Gualberto: You're a son of a bitch. You gave your employees a hard time, making them work on holidays.
Benito: You don't treat your employees any better. They complain that you don't give two shits, and deny them sick days whenever they ask.   

Here is a great example of a ranchera, interpreted by Juan Gabriel, who's sometimes referred to as the Mexican Prince, after the Minneapolitan artitst.  It should be clear that Juan Gabriel no canta mal las rancheras.