Plain English Meaning
It is the setting of the mixture control that turns off the fuel supply to the engine, causing the engine to stop running.
Full Definition
The position of the mixture control that fully cuts off fuel flow to the engine, stopping combustion. Moving the mixture control to idle cutoff leans the fuel-air mixture to zero, which shuts the engine down without using the ignition switch.
Context Anchor
Seen in engine shutdown procedures and in emergency or rejected-takeoff checklists when the pilot needs to secure the engine.
Derivation
Combines '' (the lowest stable engine ) with '' (complete of flow); the phrase describes the mixture control setting that halts fuel delivery even when the is at idle.