also: いとさめ
noun
drizzle; light rain; fine rain
A literary or poetic term for very fine, mist-like rain. Not used in everyday conversation; 小雨 (こさめ) or 霧雨 (きりさめ) are the usual words.
糸雨が静かに街を濡らしていた。
A fine drizzle was quietly wetting the town.
Compound of 糸 (thread) and 雨 (rain), suggesting rain as fine as threads. The reading しう is the standard Sino-Japanese on-reading; いとさめ is a rare kun-reading variant.