taru-adjective, adverb taking the 'to' particle
incessantly falling; prolonged continuation
A literary, somewhat archaic expression describing rain, snow, or something else that continues without pause. Often used in classical or poetic descriptions. The word is a taru-adjective/adverb taking と, so it typically appears as 霏々として or 霏々たる.
雨が霏々として降り続く。
The rain falls incessantly.
霏々たる雪に閉ざされた山道。
A mountain path shut in by unceasing snow.
しとしと describes gentle, steady rain, while 霏々 implies a heavier, more relentless downpour or continuation, and is much more literary.
絶え間ない is the common modern word for 'incessant' or 'uninterrupted'. 霏々 is its rare, poetic equivalent.
The kanji 霏 means 'falling rain or snow' and is repeated for emphasis. The reading ひ is the on'yomi. The exact historical derivation is uncertain, but it is a classical Chinese-derived literary term.