incompatible; like oil and water
Idiomatic expression describing two things or people that do not mix well, just as oil and water separate. Often used in the pattern 〜は油に水だ or 〜と〜は油に水のようだ.
He and she are like oil and water; they're always fighting.
自由奔放な彼と几帳面な彼女は、まさに油に水だ。
The free-spirited him and the meticulous her are truly incompatible.
From the literal image of oil and water not mixing, used metaphorically for incompatible people or things.