expression, noun-modifying (irregular)
flat; insipid; lifeless; listless
Describes something that has lost its fizz, energy, or spirit. Often used for drinks that have gone flat, or for a person's expression, atmosphere, or performance that feels dull and lacking vitality.
See also: 気が抜ける (きがぬける)
Flat cola tastes bad.
He sat there with a listless expression.
From 気 (spirit, energy) + の (possessive particle) + 抜けた (past/prenominal form of 抜ける 'to come out, to escape'), literally 'with the spirit having escaped'.