halberd
桙 centers on a halberd, a pole weapon with a blade. Its meaning is specific to this weapon type.
桙 combines 木 (tree/wood) with 牟, which likely contributes the sound. The character likely originally referred to a wooden shaft or a type of halberd made from wood.
The left side 木 suggests wood, and the right side 牟 looks like a cow 牛 with a mark 厶. Imagine a wooden halberd used to prod cattle.
For ウ, imagine the sound of swinging a halberd through the air: 'oooh' -> ウ. The whoosh of the wooden weapon cues the reading.