Reply #142: No, a convicted felon is not illegal.

[Igel]

You can't describe really people, just actions, acts, and states, with the adjective. "Illegal alien" is a counterexample, but such examples are rare; in this case, I suspect it's because the word 'alien' derives from an adjective (adjectives express states), and we were stuck with the lexicalized expression when sci-fi writers convinced a generation of semi-literate Americans that 'alien' means 'somebody from outer space.' Adjectives + nouns frequently have the adjective modifying the action the noun expresses; the construction is ambiguous.

Paraphrase the expressions using 'illegal'; you'll see what I mean. An illegal immigrant, for the 15th or 20th time, isn't an illegal person who immigrates; that would imply they were illegal in Mexico or Russia--instead an illegal immigrant is a person who illegally immigrates. It works for eventives, too: a quick run isn't a quick instance of running, but an instance of quick running.

Same for things like 'unauthorized resident' or an 'sloppy eater': it's not an authorized person who resides in a place, or a sloppy person that eats. Without the willingness to actually make an effort to resolve ambiguity, there's little chance for communication. Some people like spite and taking truthy nifty slogans as truth.