“把某人扔下公車”是什么意思？

在各類競爭場合，為了獲得私利，總有人會出賣隊友、犧牲朋友。這種行為就像大家同坐在一輛車上趕往目的地，他們卻選擇中途把自己的同伴推下車。

To underbus means to unfairly discredit, blame, or dispose of an ally or colleague, particularly for personal gain.
Underbus指為了個人私利，不公平地詆毀、責難或者拋棄同伴或者同事的行為。

This verb is a tidy shortening of the longer idiomatic phrase “to throw (someone) under the bus,” which has been in the language since at least 1991:
這個詞是短語“to throw (someone) under the bus”的簡寫形式，意為：將某人推下水，即為了個人利益犧牲或者背叛別人。這個短語至少在1991年時就開始使用了：

Dees said he talked to Hood after he bonded out of the El Paso County Criminal Justice Center on Sept. 26, 1990, and warned him “that he was being thrown under the bus by Jennifer Reali.” But he said Hood believed Reali “was going to tell the truth.”
迪斯說，1990年9月26日胡德從埃爾帕索縣刑事司法中心保釋出來后，他曾經警告過胡德“詹妮弗會讓他背黑鍋”。可是，胡德說他詳細詹妮弗會“實話實說”。

—Erin Emery, “Hood talks without thinking, friends testify at murder trial,” Gazette Telegraph, December 12, 1991

For example:

You can’t underbus key staffers during a campaign. Can you? May as well underbus yourself.
你不能在活動期間背叛骨干員工，對不對？沒準最后會把你自己也拖下水呢。

（中國日報網英語點津 Helen）