go back on 背信;背弃;出卖;违约;背叛

go back on one's word食言

He never goes back on his word.

他从不食言。

He would rather die than go back on his friends.

他宁死也不出卖自己的朋友。

A:Something must be wrong with Bob recently.

最近Bob有点不太对劲。

B:Yeah,he's in a bad mood,coz some of his friends have gone back on him.

是的，他心情不好，因为他的一些朋友背叛了他。