A light, porous volcanic stone used for scrubbing or as a building material.
I scrub the calluses on my heels with a pumice stone.
loose rock; loose stone
A rock or stone that is not firmly fixed, often found on slopes or in construction contexts.
floating group; group without a base
In the game of Go, a group of stones that lacks a secure base and is vulnerable to attack.
Be careful, as there are many loose rocks on the mountain trail.
Those black stones are a floating group, so they are easily attacked by White.
Compound of 浮き (uki, 'floating') + 石 (ishi, 'stone'). The exact historical derivation is uncertain, but the meaning is transparent from its components.