Solid means completely. "Frozen / freeze solid" is a common expression. It means that ice is not slushy (with water) or thin.. You can skate safely on a pond only after has frozen solid. Adverb is most likely as part of speech, since I can substitute "completely" and have the same meaning.
Frozen stiff is different from frozen solid. If Ihave been outside in cold weather, and come inside, I say, "I'm frozen stiff. " It means that my fingers don't bend very easily. Frozen solid means "all the way through". If I were frozen solid, then I would be dead. Actually, being frozen solid is a service you can buy. The theory is that if you die from a disease, and are frozen s