lack, gap, fail
缺 centers on an absence or break in something that should be whole: a lack, a gap, or a failure where something is missing.
缺 combines 缶 (pottery) with 夬, which likely contributes the sound and the idea of breaking or being incomplete. The character originally referred to a broken or chipped pot, extending to any lack or gap.
A pottery jar (缶) with a gap (夬) in its side: something is missing, so it fails to hold water. That is a lack.
For ケツ, imagine a potter finding a gap in a jar and exclaiming 'Kets!' in frustration: Kets -> ケツ, and the gap means the jar is lacking.