calidus

Calidus, calida, calidum is first and second declension, from caleo, calere, "to be warm". Its opposite is frigidus, and in the baths the pair names the rooms: caldarium and frigidarium.

Everyday speech syncopated it to caldus, and that shorter form is what Italian caldo and French chaud inherited. Spanish and Portuguese went elsewhere for "hot", taking caliente and quente from calens, calentis, the present participle of caleo. Their caldo is a real word too, but it means broth, so do not read it as "hot". English got almost nothing of it directly. "Calid" is not a word you can lean on, and the descendants that did survive hide the root well: cauldron and chowder both go back to caldaria, a cooking pot, calorie to calor, and nonchalant to a French verb chaloir that once meant "to matter" because it once meant "to be warm".