Noun
(horticulture) The first growth of buds, flowers, seeds, or whatever is harvested from a plant.
(by extension) The beginning; the period of time when something is first starting.
(wastewater management) The flow of wastewater at the start of a storm or season, when runoff has the highest concentration of pollutants.
Source: en.wiktionary.orgThe world of art was less fortunate. Many of the younger men barely lived through the first flush of youth. Destroying Death is the worst enemy to the arts. Wynford Dewhurst
This book is past the first flush of youth. It is a book that is in puberty. It is hesitating, and from the vantage point of the mature reader, it is both a sad and amusing reminder of the part which is not always attractive enough to be revisited. Peter Greenaway