in the first flush
in the first flushin 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).
1997 Tom Petsinis The 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.