na-adjective, noun
not serious; frivolous; insincere; unsteady
Describes a person, attitude, or behavior that lacks earnestness, commitment, or sincerity. Often used for someone who doesn't take things seriously, is playful when seriousness is expected, or is unreliable.
彼は仕事に不真面目だ。
He is not serious about his work.
不真面目な態度をとると、信頼を失うよ。
If you take a frivolous attitude, you'll lose trust.
Formed by prefixing 不 (fu-, 'not') to 真面目 (majime, 'serious, earnest').