complete incompatibility; like water and oil
A literary yojijukugo describing two people or things that are fundamentally incompatible, like water and fire or ice and hot coals. Rare in everyday speech; more common in written or formal contexts.
Those two are as incompatible as fire and water.
A literary compound pairing 水 with 火 and 氷 with 炭, two conventional images of mutual incompatibility.