出尔反尔 --------- go back on one's word

例句：
A: How come he doesn't show up yet? He promised to come to the party.
为何他还没出现?他答应要来参加宴会的。
B: Don't trust him. He often goes back on his word.
别相信他。他经常出尔反尔。

同义词：
blow hot and cold