in the first flush in a state of freshness and vigour.

☞ The exact origins of flush as a noun are unknown; early senses share the idea of a sudden rush or abundance of something (e.g. water, growth of grass, or emotion).

1997Tom PetsinisThe French Mathematician A month ago, in the first flush of enthusiasm…I tackled the classic problem of trisecting an angle using only a compass and straightedge.