Sometimes when I teach my Spanish classes, students will ask me how to swear in Spanish. They express disappointment when I inform them that I will not teach them how to swear in Spanish. However, whenever I ask who knows how to swear in Spanish, at least half of the students always raise their hands. “If you want to learn how to swear in Spanish, see these students after class,” I tell the inquisitive student.
Occasionally, students will ask me about certain words they heard someone saying, but then couldn’t find in their English-Spanish dictionary for some strange reason. These words are invariably profanities. The word “güey” is a common topic in class because some Mexicans use it so often–so frequently, that students think that it’s okay to use it in any context. However, under the wrong circumstances, “güey” is an insult that could result in physical abuse to the speaker. Literally, “güey” means ox. That doesn’t really sound so insulting, does it? In Mexico, “güey” has been used as an insult for so long, that it no longer even refers to an ox. To put it in perspective, think of the word that refers to the female dog: “bitch.” That word has become so offensive that I would never call any self-respecting female dog a bitch. It’s that bad! So, remember: “güey” has the same negative connotations as “bitch.”
A few years back, the Mexican restaurant Chi Chi’s had a radio commercial with the following dialogue: “No way!” “Yes way!” “No way!” “Yes way!” And when the commercial was over, the announcer said, “By the way, never say way to a Mexican.” But I’m sure he really meant, “By the way, never say “güey” to a Mexican.” I think that’s good advice everyone should follow!
Chicago USA 
The word for ox in spanish is buey. “güey” is a distortion of this word.
http://es.wikipedia.org/wiki/Guey
Personally I believe that not teaching students a few swear words can be counterproductive.
Firstly students are always going to find a way to learn this type of things, it would be probably better to learn them from a teacher than with an on line translator. You could give them insight on the use, context and intensity of the word.
Swearing is an important part of a language such as Spanish and has much to do with understanding Spanish culture.
A simple word like “cojones” is a very good example. Who hasn’t heard a Spaniard talk about how big his/someone else’s balls are referring to how brave someone is?
uses
No hay cojones: you don’t have the balls/guts
cojonudo: fantastic
¡cojones!: for f**k sake
y un cojón: no way
tiene los cojones pelados de tanto trabajar : “the skin of his balls is starting to peel from so much work” ie he works alot
acojonado:afraid i.e Estaba acojonado. Pensé que nos iban a pillar.
Tiene los cojones como el caballo de Esparteros: popular spanish saying meaning “his balls are as big as esparteros’s horse’s balls”. He is very brave(manlike). The caballo de esparteros refers to a statue in Madrid of general Esparteros. (http://es.wikipedia.org/wiki/Estatua_de_Espartero_%28Madrid%29)
As you can see, this single word not only has many meanings but it also has cultural connotation. All this would have been lost if we were simply to look “cojones” up in a dictionary.
The language teacher that taught me not to swear was the one who asked the class to listen to non-native speakers try to swear.
In halting accents, the curses made the speaker more embarrassing than forceful.