No, it is a simple question. This is not a slang for anything or an idiom for something else. This person wants to know where you're going to eat - maybe they want to know a good restaurant or maybe they just want to know more about you.
The sentence is not, because an idiom is usually a phrase. The idiom is "do lunch," which means to have lunch together, usually while discussing business.
Let's do lunch some time next week.
No,it is not an idiom. It means exactly what it says - "if the job is going to get done" with the implied ending of "I will have to do it."
"Your head is going to explode" IS an idiom. It means you have too much to think about.
An idiom is a phrase that makes no sense unless you know the idiomatic definition. Do you think that laughing would really kill you? No, so this is an idiom. It just means he laughed very hard.
The idiom of going to the dogs means that any person or thing has come to a bad end, been ruined, or looks terrible.
Phil doesn't know what's going on
The object of the preposition "for" in the sentence is "lunch." It shows the purpose or destination of the action of going.
Fair play is not an idiom - it means exactly what it says. Things are going along fairly.
It's not an idiom. It means just what it says -- it's either going to rain that day, or it will be sunny.
No, "you are going slower than a turtle" is not classified as an idiom. It is a direct comparison used to emphasize someone's slow pace.
Nothing. However, a brown bag LUNCH means one that you pack yourself, usually carried in a paper sack.