English has words with spaces. Boiling water isn't one of them, but in general, if you can't insert another adjective between an adjective-noun pair, it's linguistically a compound word that we happen to write with a space. "Fast food" is a good example. It's not simply an adjective-noun pair, as demonstrated by the fact that you sound like a crazy person if you try to insert literally anything between "fast" and "food" in "I eat too much fast food". The "fast food" can be modified all you like, as in "I eat too much lukewarm fast food", "I eat too much depressing fast food", but you can't treat "fast" as merely an adjective of "food", else "I eat too much fast, filling food" wouldn't strip the sentence of the implication I eat at McDonalds or whatever.
dirtikiti|2 days ago
Just because the phrase is used colloquially to describe a specific group of restaurants doesn't change the fact the phrase "fast food" is comprised of two words, one being a noun; the other an adjective.
Fast greasy food. Fast disgusting food.
Fast food is not a word. It is a phrase.
Words don't have spaces.