狠 (hěn) means “ruthless; fierce; ferocious; determined; to harden (one's heart); old variant of 很”. It is an HSK 6 Chinese character in the HSK 3.0 vocabulary.
他下定决心狠狠地努力一把。
Tā xiàdìng juéxīn hěn hěn de nǔlì yì bǎ.
He was determined to make a fierce effort.
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