The phrase “beggars can’t be choosers” is most often used when someone asks for assistance. It means that, when asking for help, the requester can’t be picky about what is received. "Beggars can't be choosers" can also mean that when someone is desperate for services, goods, or financial support, he or she can’t be selective in what to accept or what not to accept. It may also be used as an admonishment to someone receiving help if he or she questions what is being given and means not to question a gift or assistance if in need.
Dating back to at least the 16th century, the phrase came into use and is first recorded in writing at about the same time as an idiom with a similar meaning. “Don’t look a gift horse in the mouth” is the English proverb that mirrors “beggars can’t be choosers,” but doesn’t have exactly the same meaning. The former phrase means not to examine something freely given. It comes from the practice of looking in a horse’s mouth to determine its health and age.
“Beggars can’t be choosers” certainly carried the warning that a person ought not to be critical of a gift that is given, but it speaks more of being in a desperate situation and asking for assistance, while the phrase “don’t look a gift horse in the mouth” could be used to describe a gift that wasn't requested or even needed. At the time that “beggars can’t be choosers” came into common use, there was no welfare system for those who fell on hard times, were unable to find work, or were unable to work because of a physical disability. Beggars were common in villages, cities, and even rural areas.
These beggars were desperate and had to accept whatever assistance, food, or shelter was offered. They couldn’t be picky about turning down one offer in the hopes that a better one would come along or in turning down an offer because the gift did not meet their expectations. People of the time would have been familiar with beggars and their desperate plight and might even have uttered the phrase with a literal meaning when offering a beggar food that was slightly spoiled or a night’s rest in the barn among the animals. As the phrase's meaning would have been familiar to nearly everyone, it entered into figurative use to describe other situations in which help was offered, even if it wasn’t exactly the type of the quality the requester had hoped for when asking for assistance.