One of the most common ways these slang words came about was by reversing letter orders in the word. This is known as vesre which comes from the Spanish word for reverse, revés.

So for example the Spanish word for coffee, café, became feca.

Doctor transforms to tordo.
Chambomba is the reverse of bombacha, which are women’s panties.
Garpar is a verb meaning to pay and comes from the Spanish word pagar.
Gomía is amigo, or friend.

But not all the words that are not vesre include:

Cana for police

Afanar means to rob or steal

Guita is money

Laburar is to work

buchón - snitch, informer to the law (from the French bouillon)

chochamu - young man (vesre for muchacho)

fiaca - laziness, or lazy person (from the Italian fiacco "weak")

gomías - friends (vesre for amigos)

guita - money

lorca - hot, as in the weather (vesre for calor "heat")

mina - an informal word for woman

percanta - a young woman

pibe[4] - like "kid", a common term for boy or, in more recent times, for young man

quilombo - racket, ruckus, disorder, mess; also slang for brothel (from the Kimbundu word kilombo).

cerebrar - to think something up (from cerebro, "brain")

engrupir - to fool someone (origin unknown, but also used in modern European and Brazilian Portuguese slang)

garpar - to pay with money (vesre for pagar "to pay")

junar - to look to / to know (from Caló junar "to hear")

laburar - to work (from Italian lavorare "to work")

manyar - to know / to eat (from the Italian mangiare "to eat")

morfar - to eat (from French argot morfer "to eat")

pescar - to know (vesre from the Italian capisce "do you understand?")

If you are lost in translation you can download this free lunfardo - english dictionary

 http://www.freelang.net/dictionary/lunfardo.php#download

But who better than the son of Edmundo Rivero to explain what lunfardo is all about

En este hermoso país que es mi tierra, la Argentina,
la mujer es una mina y el fueye es un bandoneón.
El vigilante, un botón, la policía, la cana,
el que roba es el que afana, el chorro un vulgar ladrón,
al zonzo llaman chabón y al vivo le baten rana.

La guita o el vento es el dinero que circula;
el cuento es meter la mula, y al vesre por al revés.
Si pelechaste, tenés, y en la rama si estás seco.
Si andás bien, andás derecho; tirao, el que nada tiene,
chapar es, si te conviene, agarrar lo que está hecho.

El cotorro es el lugar donde se hace el amor.
El pashá es un gran señor que sus mangos acumula.
La vecina es la fulana, el tordo es algún doctor,
el estaño un mostrador donde un curda se emborracha,
y si es que hacés pata ancha te la das de sobrador.

El que trabaja, labura; quien no hace nada es un fiaca,
la pinta es la que destaca los rasgos de tu apostura.
Mala racha es mishiadura, que hace la vida fulera.
La cama es una catrera y apoliyar es dormirse.
Rajar o piantarse es irse, y esto lo manya cualquiera.

Y que te van a contar, ya está todo relojeado.
Aquello visto, es junado, lo sabe toda la tierra.
Si hasta la Real Academia, que de parla sabe mucho,
le va a pedir a Pichuco y a Grela, con su guitarra,
que a esta milonga lunfarda me la musiquen de grupo