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Jyutping sei3 gang1
Pinyin sì gēng

Definitions (Wiktionary)
  1. noun, archaic
    fourth watch (between 1:00am and 3:00am)

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Jyutping sei3 gaang1
Pinyin sì gēng

Definitions (CC-CEDICT)
  1. fourth of the five night watch periods 01:00-03:00 (old)