go back on one's word其实这个词组，我们可以把它想象成是“回到了某个人说的话上”，从而来把它引申为是“一个人只说话，不干事”来把它理解为是“食言、言而无信、违背诺言”的这层含义。

【例句】
If you promised to help，you will not go back on your word.
如果你答应了帮忙，就不要食言。

He had agreed to help me with some shopping days ago, but went back on his word yesterday. He is always acting like that.
他前几天明明答应了帮我买东西，但昨天又突然变卦了，这个人总是这样。