unenthusiastic; lacking motivation; half-hearted
Describes a state of low motivation or reluctance to do something. Often used in the form 気乗り薄な様子 or 気乗り薄そうに.
He replied unenthusiastically.
He attended the meeting with a half-hearted look.
乗り気 means 'eager' or 'enthusiastic', the opposite of 気乗り薄.
Compound of 気乗り (motivation, interest) and 薄 (thin, weak). The exact historical derivation is uncertain, but the meaning is transparent from its components.