The English idiom “go south” has a distinct meaning related to something going wrong or worsening. For instance, an English speaker may say that a deal is about to “go south,” meaning it is headed toward failure, or that profits are “going south” meaning that profits are decreasing. Speakers might also use the alternate phrase “heading south,” for instance, saying that efforts at reviving a company or product seem to be “heading south,” or dwindling.
Generally, the use of this idiom is a somewhat lighter way to talk about negative trends. It can be substituted for harsher language with words like “terrible,” “dire,” or “catastrophic.” By contrast, if someone hears someone say that something is “going south” or might “go south,” the listener usually gets the sense of urgency, but the potential negative result seems somehow not quite as bad as it might be.
In terms of the origin of this phrase, word historians have some pretty interesting thoughts on why English speakers have started to use the phrase. One idea is that profits or sales numbers are good when they rise toward the top of a chart, and bad when they flow toward the bottom. When the chart is placed on the wall, worsening numbers can seem to be, in effect, heading southward in comparison to the normal orientation of a map. An additional explanation of this phrase, which seems largely to be an American term, is the idea that after the American Civil War, the South seemed to be associated with negatives, at least form the perspective of Northerners, though few other idioms exist to support this notion.
When looking at the history of the phrase, linguists can see that, in older times, British English speakers did not use the phrase “go south” and instead referred to a worsening situation “going west.” Explanations for this include the idea that the sun sets in the west, as well as stories of prisoners from London traditionally heading west to the gallows. Over time, the American form of the phrase, “going south,” seems to have dominated the other. One reason that this might have happened is that in America, largely through historic quotes like “Go west, young man!” the west has been associated with positives, not negatives. This has led to the phrase “going south” becoming a familiar one to the majority of English speakers around the world; perhaps oddly, "going north" is not typically used to indicate an improvement of circumstances.