"Armpit" in English can be translated as either sobaco or axila in Spanish. Is each term used in different regions, or are they both used across the Spanish-speaking world? What is the difference, or are they exact synonyms? If they are synonyms, which is more common?
I am adding this summary following what was discussed in Juntemos en respuestas wiki las respuestas cortas específicas de regiones / Let's use community wiki to summarize set of short region specific answers. Feel free to edit to add the term used in your country or region.
|Argentina||Sobaco is much more informal, almost vulgar. You'd never hear it in a deodorant ad, or in medical speak.|
|Bolivia||Sobaco is really vulgar. Better use axila.|
|Chile||Sobaco is also almost vulgar and very informal.|
|Colombia||Sobaco is also almost vulgar and very informal.|
|Cuba||In Cuba we use both Axila and Sobaco. Sobaco is more informal than Axila.|
|Dominican Republic||Sobaco has the same meaning and connotations that in other Spanish speaking countries. Axila is the more ethical and formal, while sobaco is more informal.|
|Ecuador||Sobaco is more informal, I think it is better to say "axila".|
|Mexico||Sobaco is vulgar and if you do use it, it is when someone has bad odor in their armpits (te huele el sobaco, hueles a sobaco). Axila is when you are actually referring to the armpit as a body part.|
|Panama||Axila is the word that we use to refer to the armpit. Sobaco is used for people from the "Guetto".|
|Peru||Sobaco is not very used, the usual form is axila for both formal and informal situations.|
|Spain||Sobaco is more informal than axila.|
|Venezuela||Sobaco is more informal yet used by the upper class. Axila is used in medical contexts.|
In Colombia both forms are used about equally. I prefer axila since is a more technical term and sobaco is perhaps used more often when referring to animals. There's a Colombian saying that goes like this:
Estoy más pelado que sobaco de rana (I don't have a dime on me.)
Again, sobaco is more colloquial and axila is more formal/technical and they refer to the same part of the body on both, animals and humans.
In the Dominican Republic, sobaco is also used to refer to someone with bad odor on his armpit. See this dictionary of Dominicanismos by Colonial Tour and Travel
If you visit the Dominican Republic and someone tells you "tú tienes sobaco," is very likely that the person isn't referring to your body parts but rather urging you to take a shower and get a hold of a deodorant as soon as possible.
In several countries the word sobaco is considered vulgar and unsophisticated.