Verified answer

"Il faut se méfier de l'eau qui dort" means literally 'Beware of still waters', but as an expression it's more equivalent to 'The quiet ones are always trouble' or 'The quiet ones are the worst' in English.

The real translation is "Still water run very deep".

It's a corresponding idiom so the French version can't be translated word by word in English. What French mean by "Il faut se mefier de l'eau qui dort" is the same as what English mean by "Still water run very deep". There is no other translation!

the literal translation would be, one should mistrust the water which sleeps ... that sounds ridiculous, because actually this is the equivalent of the English saying " Still waters run deep".

Beware of still waters

It's French.

"Beware, still water !"