expression, godan verb (-mu)
worry without reason; fret over nothing
Idiomatic expression meaning to worry needlessly, often about things that are not worth worrying about. The repetition of 気 (spirit/mind) emphasizes the self-inflicted nature of the anxiety.
彼はいつも気で気を病んでいる。
He's always worrying himself sick over nothing.
そんな小さなことで気で気を病むな。
Don't fret over such a small thing.