What does "even-keeled" mean?
If someone is described to be even-keeled, what does that mean?

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Answer

It is from a nautical term.  Keel is part of a ship. If a ship is level and evenly balanced in the water then it is even-keeled.
If a person is even-keeled, then they are of a calm temperament.

Answer

From a thesaurus:

.....calm, composed, even-tempered, imperturbable, peaceful, placid, serene, stable, steady, unexcitable, well-balanced....

The image is one of a boat which stays steady on the water.