Glossary » mythical man-month
a man-month is a way of measuring work output for production which is reasonably helpful in certain limited areas of industries like manufacturing. The basic idea is that if you want to produce twice as many sprockets, then you can hire twice as many people to get it done. Worker productivity is measured by how much one person can do in one month. Thus, what takes 1 person 100 months to perform should take 100 people 1 month to perform.
This perspective is called “mythical” because it doesn’t apply in many organizational settings, and in particular in software development environments. Doubling the people on a software development project may not merely fail to double the speed of development, it may actually slow a project down.