it is hard to explain; it is hard to say
A set phrase used when something is difficult to put into words, often because it is complex, delicate, or beyond easy description. Has a slightly literary or formal feel.
My feelings at that time are hard to put into words.
The charm of his work is difficult to describe.
A set phrase combining 曰く “circumstances or underlying reason” with 言い難し, the classical terminal form meaning “hard to say.”