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.
"Your head is going to explode" IS an idiom. It means you have too much to think about.
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."
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.
The object of the preposition 'for' is lunch.
Phil doesn't know what's going on
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, when you say that something is "AS ___ AS ___" you are dealing with A Simile
The idiom, "You lost your marbles," means that you've gone crazy.