出爾反爾
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.
中英俚語俗語來源:孟子:“戒之戒之,出乎爾者,反乎爾者也。”
同義詞:朝令夕改 The law is not the same at morning and night.
反義詞:一言為定 A bargain is a bargain.