insouciant

他叼着香烟，以一副漫不经心的样子走进会议室。

He walked into the meeting room with an ___insouciant manner, a cigarette dangling from his lips.

那个漫不经心的继承人塞给服务生一叠皱巴巴的钞票当小费，不数找零扭头就走了。

The ___insouciant heir tipped the waiter with a crumpled stack of bills and walked away without counting the change.

当老师把不及格的试卷发还并建议补考时，那个漫不经心的学生只是耸了耸肩。

The ___insouciant student merely shrugged when the teacher returned his failing test and suggested a retake.