yodan verb with 'ku' ending (archaic), intransitive verb
occur repeatedly; be repeated
Archaic intransitive yodan verb meaning that something happens again and again. In modern Japanese, this meaning is usually expressed by 繰り返される or 頻発する.
In classical Japanese, 頻く was used to express that something like rain occurs repeatedly.
incessantly; without stop
The historical derivation is uncertain; the kanji 頻 is conventionally associated with the meaning of repetition.