Having a hearty appetite; eager to eat a lot. Describes someone with a strong appetite (a na-adjective).
あの子は食欲旺盛で、何でもよく食べる。
That child has a hearty appetite and eats everything well.
夏は暑くても食欲旺盛になる人がいる。
Some people have increased appetites in summer even if it's hot.
風邪で食欲旺盛ではなくなった。
Because of the cold, I lost my hearty appetite.