Not serious; frivolous or insincere. Lacking earnestness or sincerity (commonly used in everyday speech).
彼は授業中に不真面目な態度を取ることが多い。
He often behaves unseriously during class.
そんな不真面目な返事では困るよ。
That kind of flippant answer is troublesome.
仕事で不真面目だと信用を失う。
Being unserious at work will make you lose trust.
彼女は普段は明るくふざけるが、重要な場面で不真面目になることはない。
She usually jokes around, but she never becomes unserious in important situations.