The Mythical Man-Month is a term coined in a book of the same name by Frederick P. Brooks in a book describing the unique problem of software development on correction of schedule slippage. It makes special mention of the fact that unlike other engineering problems when you fall behind schedule you can't just add people to the project and expect the extra hands to resolve the problem.
This is largely due to the idea that while a laborer can be added to the digging of a hole, in software development everyone’s idea, not only of what the hole is, but also the best way to dig it can vary widely and dramatically, leading to the idea that the addition of personnel to a development team will actually cause the schedule to slip more as they are made aware of the particulars of this 'hole' and the method being used by the existing team to 'dig' it.
The book can be considered a classic text of software engineering. It was republished virtually unchanged (just an addition of a chapter) after 20 years.