expression, noun
long face; looking dejected
Describes a facial expression that shows unhappiness, worry, or disappointment. Often used in set phrases like 浮かぬ顔をしている.
See also: 浮く
彼は浮かぬ顔で会議に参加した。
He attended the meeting with a long face.
浮かぬ顔をしているけど、何かあったの？
You look down. Did something happen?
浮く means 'to float' or 'to be cheerful'; 浮かぬ is its negative form, so 浮かぬ顔 literally means 'a face that is not cheerful'.
From 浮かぬ (negative of 浮く, 'to be cheerful') + 顔 ('face'). The expression is a fixed phrase meaning 'a gloomy face'.