Definition
The propeller blade angle setting at its smallest (flattest) position relative to the plane of rotation, producing the lowest pitch and corresponding to the highest engine RPM for a given power setting. On a controllable-pitch or constant-speed propeller, full low pitch is selected by pushing the propeller control fully forward (high RPM).
Plain English
The propeller blades are turned to take the smallest 'bite' of air. This lets the engine spin at its highest allowable speed, which is what you want for starting, takeoff, and hand propping.
Context Anchor
Seen in engine-start and hand-propping procedures for airplanes with a propeller control.
Derivation
Pitch' here refers to how far a propeller would advance through the air in one revolution if the air were solid — like the threads of a screw. 'Low pitch' means a small advance per turn, so the blades are nearly flat to the wind. 'Full' simply means all the way to that limit.
Why Pilots Care
Reduces propeller drag so the engine can develop full RPM during starting and takeoff.
Intuition Check
Low pitch does not mean low engine speed. Here, low means a low blade angle; full low pitch is the flat-blade setting normally used for starting.
Example Sentence 1
Before hand propping the aircraft, the pilot confirmed the propeller control was set to full low pitch.
Example Sentence 2
With the prop at full low pitch the engine reached 2400 RPM during the magneto check.