in the first flush
in the first flush of (something)
In the earliest of beginning stages, especially when energy, effort, or enthusiasm are at their peak. Of course they're happy now—they're still in the first flush of marriage! Let's see how they're doing 10 years down the line. The novel consists of parallel storylines: one follows the protagonist in the first flush of adulthood, while the other examines how his views have changed after a lifetime of experience and hardship.
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.