I am Mexican.. Everybody told you what the phrase literally means, but the context is wrong.. It is normally used when somebody is speaking a lot to you, and you are not really interested or paying attention on what the other one is saying, so you just answer "No pos ta cabrón"
Yes, what you read is slang and what I wrote above would be the official version. Since you saw it in a relationship meme, im sure it is meant as "this is a very difficult situation" or "that´s tough". (I am Mexican) Hope that helps :)
This expression is used a lot by Mexicans but also by people in Central America in general. People from Spain may also say it and would of course understand what you mean. The slang changes the farther south the country is in Latin America. Colombia (and countries nearby) would already have a different slang and Argentina would have again a completely different one from the Colombian. Glad I could help.
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