Can an adjective "robust" be a noun in a sentence? And if it can't how would you say with one word "robust fellow" that can be applied to both man and woman? Because as I understand "robust fellow", we say about a man.
"robust" is an adjective and it could only be used as a noun when you refer to a class ("the robust") just as you say "the poor", "the rich", "the wealthy". Other adjectives that have a similar meaning are "sturdy", "able-bodied" and "stocky".
- robust (adj) - strongly formed or built. MW
- stocky (adj) - compact, sturdy, and relatively thick in build.
- sturdy (adj) - strong and healthy
These words can be used for both males and females, but I don't think a woman would like to be referred to as "stocky" or "robust".
I would use "stout."
: thick and strong
: physically or materially strong
You could also use "hearty."
: strong, healthy, and active
Definitions from Merriam-Webster