To Pigeon Hole a person is to classify them and give them a specific identity when more than one might be more appropriate. The English used to keep pigeons as domestic birds, although not as pets, but for food. Pigeons generally do not stray too far from a place they are being fed, so folk would set small openings into walls, or build boxes with recesses that pigeons would naturally make their home, without realising they would later be eaten. These were known as pigeonholes.
During the 18th century offices would have small compartments built into the furniture to file documents and, owing to the resemblance, these also became known as pigeonholes. These pigeonholes would each have categories and the documents in them would all have a similar theme, decided by the filing clerk. When a document had more than one reference it was up to the clerk to decide where it was to be placed and hence a document was ‘pigeonholed’.