His suggestion was shot down by the boss in front of everyone at the meeting.

shot down 是动词短语 shoot down 的被动形式。shoot 本意是"射击"，down 表示"向下"，合起来字面意思是"击落"。 在这里用作比喻意义，表示"断然拒绝"或"严厉驳斥"，就像用枪击落飞机一样彻底否定某个提议。