pike, windy-again radical (no. 79)
殳 centers on a long-handled weapon: a pike or halberd-like polearm. The radical sense is the same weapon shape used as a component in other characters.
殳 is a pictograph of a long-handled weapon, with 几 suggesting the handle and 又 the hand holding it. The exact historical development is uncertain, but the weapon interpretation is standard.
The top 几 looks like a long handle, and the bottom 又 is a hand. Picture a hand gripping a long pike: 殳 is a polearm.
For シュ, imagine a soldier thrusting a pike with a sharp 'shoo' sound: shoo -> シュ, and the pike strikes forward.
kanji radical 79 at right