Plain English Meaning
The chunky base of each propeller blade, right next to the hub. It's the part that holds the blade onto the propeller, not the part that does the work of pulling the airplane through the air.
Full Definition
The thick, rounded inner sections of a propeller blade, located close to the hub, where each blade attaches to the propeller assembly. Blade shanks are structural rather than aerodynamic — they carry the loads from the spinning blade into the hub but produce little useful thrust because of their round, untwisted shape.
Context Anchor
Seen in propeller blade-angle control discussions, especially when explaining how a propeller blade changes angle instead of staying fixed in one position.