咬人狗兒不露齒 [－－－儿－－齿]

Jyutping ngaau5 jan4 gau2 ji4 bat1 lou6 ci2
Pinyin yǎo rén gǒu r5 bù lù chǐ

Definitions (CC-CEDICT)
  1. lit. the dog that bites does not show its fangs (idiom)
  2. fig. You can't tell the really dangerous enemy from his external appearance.