jar, pot, hinge knuckle, one's aim
壷 centers on a container with a narrow opening: a jar or pot. The extended senses of a hinge knuckle (a socket-like part) and one's aim (a target, like a pot to aim at) are natural applications of that container idea.
壷 is a variant form of 壺, which is a pictograph of a lidded jar or pot. The top 士 represents the lid, and the lower enclosure represents the body of the container.
The top 士 looks like a lid, and the lower part is a wide container. Picture a lidded jar or pot. The hinge knuckle is like a socket that holds something, and one's aim is the target you want to hit, like aiming at a pot.
For コ, imagine a small pot that makes a 'co' sound when you tap it: co -> コ. The jar is so small it just goes 'co'.
funerary urn; cinerary urn; burial urn; cremation urn
plunge basin; waterfall lake; plunge pool; basin under a waterfall
individually "washing" a partner's fingers or toes in one's vagina (esp. as a service at a soapland)
charcoal extinguisher
pussy; cunt
night-soil reservoir dug in the field; field latrine
metal jar; metal pot