"笑掉大牙" (xiào diào dà yá) 是指因为某件事情非常有趣或者逗乐，而笑得牙齿都露出来了。这个表达的英文翻译可以是 "laugh one's head off" 或者 "laugh until one's teeth show"。

来源：这个表达可能来源于中文俗语 "笑得牙齿都掉了" (xiào de yá chǐ dōu diào le)，意思是因为笑得太厉害了，牙齿都掉出来了。这个俗语可能源于人们在笑得特别开心的时候，常常会笑得牙齿都露出来。

下面是一些使用 "laugh one's head off" 或者 "laugh until one's teeth show" 的例句，以及它们的中文翻译：

"I can't stop laughing, I'm laughing my head off!" (我笑得停不下来，笑掉大牙了！)
"That joke was so funny, I was laughing until my teeth showed!" (那个笑话太有趣了，我笑得牙齿都露出来了！)
"I couldn't stop laughing at the comedian's jokes, I was laughing my head off the entire time." (我笑话讲得很有趣，我整个时间都在笑掉大牙。)
"We all laughed until our teeth showed when the clown fell off the tightrope." (当小丑从高空绳索上掉下来的时候，我们都笑得牙齿都露出来了。)