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Jyutping zaat3 maang5 zi2
Pinyin zhā měng zi

Definitions (CC-CEDICT)
  1. to swim with head submerged
Definitions (Wiktionary)
  1. verb
    to dive headfirst into water; to swim with your head underwater
    (syn.) 打猛子, 打沒頭壇, 沉沬, 打没头坛