1. They arrived unbidden at the midnight.
    他们半夜里不期而至。

2. He spoke out unbidden at the meeting.
    他在会议上主动发言。

1. 未受邀请的：paleotemperature (海洋的)古温 | unbidden 未受邀请的 | pneumonics 压气(射流自动)学

2. 未受指使的：unbidden 未受邀请的 | unbidden 未受指使的 | unbitted 不受约束

3. 未奉命的：unbiasedness 不偏性 | unbidden 未奉命的 | unbidden 未受邀请的

4. 未奉命的/未受指使的/自愿的/未受邀请的：unbiassed /无偏见的/公平的/ | unbidden /未奉命的/未受指使的/自愿的/未受邀请的/ | unbind /解/解开/解放/

1. 自发的；不由自主的
    If something happens unbidden, it happens without you expecting or wanting it to happen.

    e.g. The name came unbidden to Cook''s mind — Ashley Stoker...
           库克的脑海里不由自主地闪现出一个名字——阿什利·斯托克。
    e.g. Unbidden, his thoughts turned to his wife, who had died two years ago.
           他不禁想到了两年前去世的妻子。