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

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

beware of a silent man and still water

US /bɪˈwer ʌv ə ˈsaɪlənt mæn ænd stɪl ˈwɔːtər/
UK /bɪˈweər ɒv ə ˈsaɪlənt mæn ænd stɪl ˈwɔːtə/
"beware of a silent man and still water" picture

Idiom

a warning that people who are quiet or calm can be more dangerous or complex than they appear

Example:
He doesn't say much in meetings, but beware of a silent man and still water; he's likely planning his next move.
I never expected him to lead the rebellion, but they say beware of a silent man and still water.