Why is it "rough-looking" instead of "roughly looking"?

I’ve noticed recently that certain constructions with present active participles (meaning, -ing forms acting adjectivally) prefer to be modified by adjectives rather than by adverbs.

For example, it sounds better to say:

• a rough-looking neighborhood

than it does to say:

• a *roughly looking neighborhood

even though rough, an adjective, is clearly modifying looking, another adjectival word.

Is there any rule that explains this? Or is this just a strange corner case of English?

Solution 1:

It’s because sense verbs like look, sounds, tastes, smells, and feels customarily take adjectives rather than adverbs. Notice how normally it is good that goes with each of them, and how using well there would mean something altogether different.

• She really looks good in that new outfit.
• That only sounds good over the headphones.
• Boy does this ever taste good!
• Last week’s garbage doesn’t smell so good.
• A good night’s sleep always feels good.

Now go back and swap in well for each good in the five example sentences above. In all cases it changes the meaning significantly, and in most cases it is sheer nonsense.

This is even more apparent if you try it with poorly.

Solution 2:

It's impossible to divorce syntax from meaning here, though some try to. I've seen the word 'modify' given very strange and forced shades of meaning to ensure that the definition of a central adverb as a word that modifies a verb 'works'.

However, let's start with a couple of easy cases:

Contrast:

The teacher marked the question wrong.

The teacher marked the question wrongly.

In the first example, wrong is used in secondary predication, and is an adjective addressing (I'm happy with modifying here) the (teacher's assessment of) the accuracy of the answer.

In the second example, wrongly is used as an adverb modifying marked.

Now, in the example rough-looking neighbourhood (used prenominally, a hyphen is often inserted) the neighbourhood is described as rough looking; it may or may not deserve the first impression it invokes. Rough looking or rough-looking is unitary, a compound adjective modifying the noun neighbourhood.