“把某人扔下公车”是什么意思？

在各类竞争场合，为了获得私利，总有人会出卖队友、牺牲朋友。这种行为就像大家同坐在一辆车上赶往目的地，他们却选择中途把自己的同伴推下车。

To underbus means to unfairly discredit, blame, or dispose of an ally or colleague, particularly for personal gain.
Underbus指为了个人私利，不公平地诋毁、责难或者抛弃同伴或者同事的行为。

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:
这个词是短语“to throw (someone) under the bus”的简写形式，意为：将某人推下水，即为了个人利益牺牲或者背叛别人。这个短语至少在1991年时就开始使用了：

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.”
迪斯说，1990年9月26日胡德从埃尔帕索县刑事司法中心保释出来后，他曾经警告过胡德“詹妮弗会让他背黑锅”。可是，胡德说他详细詹妮弗会“实话实说”。

—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.
你不能在活动期间背叛骨干员工，对不对？没准最后会把你自己也拖下水呢。

（中国日报网英语点津 Helen）