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.
古語では「雨が頻く」のように、同じことが繰り返し起こることを表した。
In classical Japanese, 頻く was used to express that something like rain occurs repeatedly.