still waters run deep 

A Proverb that suggests that quiet people are often very thoughtful.

Steve: I get the impression that Jill isn't very smart. She never says anything. Fran: Don't be so sure. Still waters run deep you know.

still waters run deep by LittleRatchet September 30, 2013

Still waters run deep 

An idiom and proverb of Latin origin that emphasizes that a low-key exterior hides a passionate nature or surprising qualities. Concurrently, it serves as a warning that calm or quiet people are dangerous.

In short, it means, "don't judge a book by its cover" and "there's more than meets the eye."

Example1: "My mother doesn’t talk much when we’re all gathered together, and looks very calm but, you know, still waters run deep. Before we arrive she spends the whole day rushing about, shopping, cooking, making sure everyone is coming, and without her energy nothing would happen in this family.”

Example 2: "Amanda isn't much of a talker, but who would've thought she had such an impressive talent in singing? Wow, still waters run deep."

Example 3: "A group of kids would talk shit about people in their native language. They mainly made fun of Jonathan, the skinny quiet kid. One group member decided they were gonna put their hands on him. Turns out Jonathan has been practicing Kickboxing since he was a child. Not only that, after he laid the group member out, he dresses the entire group down IN THEIR NATIVE LANGUAGE! Everyone found out he spoke three different languages. Still waters run deep, who would've thought?!"

Still waters run deep by InvaderTaylor December 17, 2022