出尔反尔
Play fast and loose.
Blow hot and cold.
Go back on one's word.

eg：Be true to what you have promised and do not play fast and loose.

中英俚语俗语来源：孟子：“戒之戒之，出乎尔者，反乎尔者也。”

同义词：朝令夕改 The law is not the same at morning and night.
反义词：一言为定 A bargain is a bargain.