Brooks' Law
The observation that adding more people to a late software project makes it even later, because new team members need ramp-up time and increase communication overhead.
Origin
Fred Brooks formulated this principle in his 1975 book The Mythical Man-Month, drawing on his experience managing the IBM System/360 and OS/360 projects in the 1960s. Brooks observed that new programmers added to a late project needed ramp-up time and drastically increased communication overhead — the number of channels grows proportionally to the square of team size. His original formulation was blunt: "Adding manpower to a late software project makes it later."
Everyday Use
You've probably seen this at work — a project is behind schedule, so management throws more people at it, and somehow the deadline slips further. Each new person needs to be brought up to speed by someone already on the team, and every additional team member multiplies the number of communication channels. It's why "nine women can't make a baby in one month."