"他们在外面吃饭"可以用英文表达为"They are eating out."

"Eat out"是一个常用的短语，意思是"在外面吃饭"。这个短语的来源是由两个单词"eat"和"out"组合起来的。"Eat"是动词，意思是"吃"；"out"是副词，意思是"在外面"。

下面是一些使用"eat out"的英文例句及其中文翻译：

I don't feel pke cooking tonight. Let's eat out.
(我今晚不想做饭。我们出去吃吧。)

We often eat out on weekends.
(我们常常在周末出去吃饭。)

Do you want to eat out or order in?
(你想出去吃还是在家订餐？)

Eating out can be expensive.
(在外面吃饭可能很贵。)