What is Throw Under The Bus?

Legal Definition
"To throw (someone) under the bus" is an idiomatic phrase in American English meaning to sacrifice a friend or ally for selfish reasons. It is typically used to describe a self-defensive disavowal and severance of a previously-friendly relationship when the relation becomes controversial or unpopular.

The phrase has been widely popularized by sports journalists since 2004 and was picked up by the mainstream media during the 2008 political primary season. It has frequently been used to describe various politicians distancing themselves from suddenly unpopular or controversial figures whom the candidate has previously allied themselves with. David Segal, a writer for The Washington Post, calls the expression "the cliché of the 2008 campaign".

In a March 2008 NPR report, the linguist Geoff Nunberg noted that "under the bus" "has appeared in more than 400 press stories on the campaign over the last six months".
-- Wikipedia