paint, decorate
墍 centers on applying a surface treatment to beautify or protect: painting and decorating. The meanings are closely related actions of covering or adorning a surface.
墍 combines 旣 (already, finished) with 土 (earth, soil). The exact historical development is uncertain, but it likely relates to finishing or coating an earthen surface.
Picture a wall of earth (土) that has already (旣) been coated with paint. The finished surface is decorated and protected.
For キ, imagine a key (キー) used to scratch a decorative pattern into fresh paint on an earthen wall: key -> キ, and the decoration is complete.