aqua silente.

Cave tibi cane muto, aqua silente. Beware of the silent dog and still water. A Latin idiom: 'Stagnant water can poison you.' 'Silent dogs can bite.' Some Romans believed that taciturn adversaries were more dangerous than the ones who made a lot of noise.
Found on http://www.wordinfo.info/words/index/info/view_unit/3592/
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