adverb
incessantly; without stop
Archaic adverb meaning 'continuously' or 'without ceasing'. Not used in modern Japanese; encountered only in classical texts.
古語では「頻く頻く」と書き、絶え間なく続くさまを表した。
In classical language, 頻く頻く was written to express something continuing without pause.
Modern しくしく is an onomatopoeia for weeping or a dull pain, unrelated to this archaic adverb.
Reduplication of 頻く (shiku), an archaic verb meaning 'to occur frequently'. The exact derivation is uncertain; the spelling is conventionally associated with the meaning.