old pond; old pool
Refers to a pond that has existed for a long time, often with a sense of age and tranquility. Commonly associated with the famous haiku by Matsuo Bashō.
庭の古池には鯉が泳いでいる。
Carp are swimming in the old pond in the garden.
Bashō's 'old pond—a frog jumps in, the sound of water' is a famous haiku.
Compound of 古 (old) and 池 (pond). The exact historical derivation is straightforward; the word is a simple noun phrase.