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Jyutping jat1 jin3 bat1 sing4 haa6
Pinyin yī yàn bù chéng xià

Definitions (Wiktionary)
  1. phrase, idiomatic
    one swallow does not a summer make
    (syn.) 孤燕不成夏, 獨燕不成夏, 独燕不成夏