To throw someone under the bus
To betray someone for selfish reasons - to advance yourself or protect your own interests
e.g. the project my work colleague handed in was full of typos (written mistakes of grammar, spelling and punctuation). When our boss complained about it, my colleague threw me under the bus: he told him that it was my job to read it through and correct any typos.
The origin of the idiom is not certain. It is likely to have stemmed from British politics (and their famous buses) where the phrase “under a bus” was already being used as a metaphor for misfortune or an ill-timed accident. A Times article in 1982 used the idiom: "The Conservative benches listened to her in silence. She was in deep trouble and the lobbies hummed with the prospect of her departure. President Galtieri had pushed her under the bus which the gossips had said was the only means of her removal."