noun
first spring thunderstorm
Refers to the first thunderstorm of spring, traditionally believed to wake hibernating insects. A seasonal word (kigo) used in haiku and weather lore.
虫出しの雷が鳴ると、春が来たと感じる。
When the first spring thunder rumbles, I feel that spring has arrived.
俳句では、虫出しは春の季語として使われる。
In haiku, mushidashi is used as a spring seasonal word.
春雷 is a more general term for spring thunder, while 虫出し specifically refers to the first thunderstorm that is said to bring insects out of hibernation.
Compound of 虫 (mushi, 'insect') and 出し (dashi, 'bringing out'), from the folk belief that the first spring thunder awakens insects from hibernation.