Meaning of the word "beware of a silent dog and still water" in English

What does "beware of a silent dog and still water" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

beware of a silent dog and still water

US /bɪˈwer ʌv ə ˈsaɪlənt dɔɡ ænd stɪl ˈwɔtər/
UK /bɪˈweər ɒv ə ˈsaɪlənt dɒɡ ænd stɪl ˈwɔːtər/
"beware of a silent dog and still water" picture

Idiom

a warning that people who are quiet or seem calm can be more dangerous or powerful than those who are loud

Example:
He doesn't say much in meetings, but beware of a silent dog and still water; he's very influential behind the scenes.
I thought she was harmless until she outsmarted everyone; beware of a silent dog and still water.