kǔ​kǔ

persistently

苦苦 kǔ​kǔ
persistently adverb
persistently; strenuously; earnestly and doggedly (as in pleading or pursuing a goal)
bitterly
painfully; in a state of misery or extreme hardship
dukkha-dukkha
(literary) a Buddhist term referring to the suffering caused by physical or mental pain, as one of the three types of suffering

Frequency

Movie subtitles
3.34 per million appears in 2% of films
Written text
7.59 per million

Sources