The term “pigeonhole” has both a literal and a metaphorical meaning. In the literal sense, a pigeonhole is a small space or recess; the term comes from the individual nesting boxes used in a pigeon loft, and it has been extended to the small compartments used in many desks and office organizers to help keep material organized and tidy. In the metaphorical sense, people use the term to talk about categorizing things and organizing them in a way which makes sense. However, pigeonholing is not always an effective organization technique, and the term is often used disparagingly.
Humans have been raising pigeons and other birds for centuries, as ample examples of historical pigeon lofts indicate. Like other birds, pigeons like to nest in secure, dark, comfortable spaces, and people typically created individual pigeonholes for each of their charges to encourage them to nest and to minimize stress. The pigeonholes could also be used to keep track of birds, as they are today, since many pigeons adopt a particular nest, allowing people to quickly count their birds and to identify who is missing by looking to see which holes are occupied.
Since the small compartments in desks and organizers resemble the holes used by nesting pigeons, the concept of a pigeonhole spread to include furniture as well. As anyone with a lot of small things to organize knows, an array of pigeonholes can be incredibly useful. For example, someone can store organized receipts in pigeonholes, breaking the receipts up by category to make them easy to find. In the sense of a compartment for holding papers and other materials, pigeonholes are quite handy.
The concept of a pigeonhole as in a category or label dates to the mid-1800s. The term is often used pejoratively, implying that the categorized items defy categorization, or that they are crammed together in a way which is not organized or logical. People sometimes also use it to refer to themselves, as in “don't pigeonhole me,” meaning that they feel unfairly categorized and suggesting that they could be employed in more wide-ranging, effective projects.
In the sense of abstract mental categories, pigeonholing often involves gross simplification and a lack of understanding about the complexities of an issue. It is also often heavily based in stereotypes, with people relying on stereotypes rather than hard information to make a decision about something.