From Latin *calma*, itself from Late Greek *kaûma* ("burning heat," hence the stillness of a midday heatwave). The word drifted from "scorching stillness" to "peaceful stillness" through Old Portuguese and Spanish.

Brazilians use *calma!* as a standalone interjection constantly — to calm someone down, to ask for patience, or even to buy yourself a moment to think ("calma aí" = "hold on a sec"). It's far softer and more common than the equivalent "relax!" in English and is rarely seen as rude. You'll also hear the phrase *com calma* ("with calm / take it easy") used to encourage someone not to rush.