distant look; faraway look
Describes a gaze that seems to be looking into the distance, often while lost in thought or reminiscing.
He had a distant look in his eyes, remembering the old days.
Gazing out the window, she was lost in thought with a faraway look.
A compound of 遠い (distant) and 目 (eye), literally 'distant eyes'. The expression is straightforward and its origin is transparent.