tub
杅 refers to a tub, a container for holding liquids or washing.
杅 combines 木 (tree/wood) with 于, which likely contributes the sound. The exact historical development is uncertain, but the character likely originally referred to a wooden tub.
A wooden (木) tub with a handle-like shape (于) on the side, ready to hold water.
For ウ, imagine a wooden tub making a hollow 'oo' sound when you tap it: 'oo' -> ウ.