Example: You want to have children! We can't afford them, we don't have a pot to piss in.
Origin: In medieval London, people did not have indoor plumbing. It was common to use a chamber pot as an indoor toilet. The chamber pot could then be dumped out a window into the street gutter below. A person who did not have a "pot to piss in" was poor indeed.
In medieval times the word "piss" was not considered at all vulgar. It was not until Victorian England that words such as piss were deemed vulgar.
Even today phrases like "pot to piss in" and "Full of Piss and Vinegar" are somehow considered to be generally acceptable and only moderately crude.