Grin一词常用来形容人“咧嘴笑”，所以短语 grin from ear to ear的意思是 “开心地咧嘴大笑，笑得合不拢嘴”。

【例句】
I grinned from ear to ear when I heard about my promotion.
听到升职的消息，我笑得合不拢嘴。

It was his birthday, so no wonder he was grinning from ear to ear!
那天是他的生日，怪不得他当时咧着嘴笑得那么开心。