Question: This has puzzled me for a few years now. When preceded by *'a'*, *shop* becomes a noun. Does "do a shop" even make sense then? The correct phrase for me was always "go shopping", or similar. 

Can anyone tell me where "do a shop" came from and whether it's grammatical?

Take the article in the following link, from an Irish broadsheet newspaper:
<http://www.independent.ie/lifestyle/parenting/i-have-seven-children-and-my-weekly-shop-includes-200-nappies-and-20-litres-of-milk-3217903.html>  

The headline, which appears to be a quote(but is not contained in the actual article body) shows an example.  

Take the following line from the article *"it takes two trolleys to carry the weekly shopping"*. If we were to replace *'shopping'* with *'shop'*, would it still make sense?

Answer: 
If you don't use this phrase, you may not know that it carries some precision. If I say that after I do X, I think I will have time to go shopping, you don't know if I just intend to randomly browse, get birthday presents, or buy the weekly groceries. If I say I think I will have time to shop, then the random browsing is excluded (among the people I talk to) by that phrasing. And if I say I have time to do a shop, it's marked as a sort of to-do list item of regular occurence, meaning the weekly "grocery shop" that has to be done at a regular rhythm. The reason we have three ways of saying the same thing is because they aren't precisely the same thing.
"Do the shop."

Usually means, "Do the weekly shop".

Grammatically, going for a shop, is like going for a walk. "Walk" can be either a noun or a verb - "to walk", "the walk".

When people say, "to go for a walk", what they mean is "to go walking" or to go on "the walk", or "the usual walk".

Do people stop to consider whether it is a noun or a verb before they say it? I don't think so. Is it understandable? Yes

Possibly it is an example of an ellipsis: 

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> (grammar, rhetoric) 2. The omission of a grammatically required word or phrase that can be inferred.
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