To underbus means to unfairly discredit, blame, or dispose of an ally or colleague, particularly for personal gain.
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.”
—Erin Emery, “Hood talks without thinking, friends testify at murder trial,” Gazette Telegraph, December 12, 1991
You can’t underbus key staffers during a campaign. Can you? May as well underbus yourself.