I came across the following sentence:
Buy yourself a little happy.
I don't understand why the adjective form happy is used instead of happiness.
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Adding to the example:
"KAREN: Yeah I know I joke a lot about hating Will and such but he's my lawyer and seeing how he affects my best friend I cant help but care too. You want to go last minute shopping? Buy yourself a little happy."
In the context of joking, the phrase "Buy yourself a little happy." is related to "You want to go last minute shopping?". As we all know you "can't buy happiness" (a very commonly used phrase), so she is making a joke by saying "Buy yourself a little happy."
It's just deliberate use of an ungrammatical form to make it stand out as "clever/catchy/memorable".
The golden rule here is Learn the rules, then break them. But I would strongly advise learners not to aspire to copy such habits. Unless you've got quite high proficiency in English, you're likely to break the wrong rules in the wrong contexts. And even if you do get it "right", people might well assume that since you're not a native speaker, you probably only managed to do so by accident. So they won't necessarily think "He knows our language well enough to "play" with it just like we do!"
In the UK, for example, the insurance company Norwich Union has an advertising campaign with the strapline "Let us quote you happy". They know it's not "grammatical", and so do we. They probably also know that partly because it's "unusual", it will remind many people of the (more grammatical) hotels.com advertising campaign Wake up happy. That's "clever" - gaining publicity from another company's spend!
I'm thinking of a deformation here. This pattern might have started as "buy yourself a little happy house" for example; and as the object is subject to change, it might have used without object in those odd times where you can't find an object to buy. Again, it is just a suggestion. Forms in every language are uniquely odd.