A stiffening chill-- is "stiffening" an adjective?

Solution 1:

Yes, it is. "Stiffening" is the present participle of the verb "to stiffen." The present participle of nearly all verbs can be used as adjectives.

http://www.edufind.com/english-grammar/ing-forms/

Solution 2:

The word stiffening in this context is indeed an adjective derived from a verb.

My English-German dictionary tells me, it is an adjective. Source

stiffening {adj}

In German, we call something like this a pseudo-participle.

Solution 3:

Clearly the use is adjectival, and the form is a verb in present participle form. I would cautiously argue this might be a gerundive, oft thought not to exist in English. I think the question is whether stiffening describes whether the chill itself is becoming stiff (in which case it's merely a participle), or whether it indicates something else is (e.g. the person the chill hits). I think the latter is the case. Therefore, at least according to this, it might be a gerundive.

On the other hand the existence of such a part of speech in English appears controversial: "There is no grammatical equivalent [to the Latin gerundive] in English, and the term is rarely used" (Oxford Dictionary of English Grammar, 2014).; in that case I would suppose it is an adjective derived from the verb taking the form of the present participle.