1. Suru verb

   to fret (over); to brood (about); to mope; to worry

   Onomatopoeic or mimetic word

   彼は過去の失敗の事をくよくよ思っている。

2. Adverb (fukushi), Adverb taking the 'to' particle

   worriedly; (fretting) constantly; (worrying) over this and that; (obsessing) to no end

   Onomatopoeic or mimetic word

Also written クヨクヨkuyokuyo