14th cent. Probably from Catalan calma "calm," "calm seas" or Italian calma 'id.,' from Latin cauma "heat of a midday sun," from Ancient Greek καῦμα (kaûma) "heat," from Ancient Greek καίειν (kaíen) "to burn." Mycenaean -ka-wo.
According to Beekes (2014), from a pre-form *κάϝω (káwo). From Proto-Indo-European *keh2-u̯- 'id.'
Attic κάω (káo), Mycenaean -ka-wo
The word was originally a nautical term for the heat during midday, and came also to mean the tranquil waters during that time.