expression, i-adjective
can't understand (someone's) mind; unable to fathom (reason/motive)
Used when one cannot comprehend another person's thoughts, feelings, or motives. Often carries a nuance of bewilderment or suspicion.
彼がなぜそんなことをしたのか、まったく気が知れない。
I really can't understand why he did such a thing.
あの人の考えていることは気が
I can't fathom what that person is thinking.