如果你想表达“带某人出去吃饭”，可以说take sb (out) to lunch。但out to lunch却没有这个意思，真正表达的是“疯狂的，注意力不集中，心不在焉，神志不清”。

例句：

You must have been out to lunch when you wrote that weird report.

当你做这个奇怪的报告的时候，你一定是神志不清的。

It seems that you were out to lunch in class.

你上课时似乎心不在焉。

I'll take you out to lunch when I get back.

我回来后带你出去吃午饭。