duty

英 ['djuːtɪ] 美['dʊti]

serve or act as a substitute for something else

代替…之用，充当

the rusting shack which did duty as the bridge.
当桥用的生锈棚屋。

engaged (or not engaged) in one's regular work

在（或不在）值班（的），在上（或下了）班（的）

the doorman had &B{gone off duty} and the lobby was unattended.
看门人下班了，大厅无人照看。

Obliged

责无旁贷的

You are duty bound to help your little sister and brother.
你有责任帮助你的弟弟妹妹