earthen jug, crock, cistern
堈 centers on a container made of earth or clay: an earthen jug, crock, or cistern for holding liquids.
堈 combines 土 (earth, clay) with 岡, which likely provides the sound. The character refers to an earthenware vessel.
The left side is earth 土, and the right side 岡 looks like a sturdy container with a lid. Picture a clay jug made from earth, shaped like 岡.
For コウ, imagine a clay jug with a wide opening, like a big crock. The word 'crock' sounds like コウ, and the jug is made of earth.