peak of spring; height of spring
A poetic seasonal word (kigo) used in haiku and traditional poetry to evoke late spring when the season is at its fullest.
春深し、山辺の桜も散りはじむ。
Spring is at its height; even the mountain cherry blossoms begin to scatter.
この句の季語は「春深し」です。
The seasonal word in this haiku is 'harufukashi' (peak of spring).
Compound of 春 (haru, 'spring') and the classical adjective 深し (fukashi, 'deep'), meaning 'spring is deep' or 'spring is at its height'. The form 深し is the classical terminal form of modern 深い (fukai).