as a noun, 'heavy' is less common and can refer to a person or thing that is heavy in weight or a person who has a specific role, especially in crime or security.
The movers struggled with the heavy and had to use a dolly.
The club owner hired a few heavies to ensure there were no disturbances.
the noun form of 'heavy' is less common and might be unfamiliar to some speakers, especially when referring to a person in the context of crime or security.
as an adjective, 'heavy' describes the weight, density, or intensity of something.
used for referring to something that weighs a lot; describing a situation or event that is serious or intense;
indicating a large amount or degree of something.
The box was too heavy for her to lift.
The movie had a heavy emotional impact on the audience.
There was heavy traffic on the highway this morning.
'heavy' can be used both in a literal sense (referring to weight) and a figurative sense (referring to intensity or seriousness).
as an adverb, 'heavy' modifies a verb, adjective, or other adverb to indicate a high degree or intensity of an action.
It was raining heavy when I left home.
He was breathing heavy after the run.
the adverbial use of 'heavy' is less formal and might be considered colloquial in some contexts. The more standard form would be 'heavily' (e.g., 'It was raining heavily').