adverb taking the 'to' particle, taru-adjective
falling heavily (rain, snow)
Describes rain or snow falling steadily and heavily, often with a quiet, persistent quality. Used adverbially with と or as a literary adjective.
雪がしんしんと降り積もる。
The snow falls heavily and piles up.
雨がしんしんと降り続いている。
The rain continues to fall steadily.
しとしと describes a gentle, drizzling rain, whereas しんしん implies heavier, accumulating precipitation.
The origin is uncertain; it is a mimetic word evoking the sound or feeling of steady, heavy precipitation.