Origin and history of warmth

Old English wearm, "having a moderate degree of heat," from Proto-Germanic *warmaz (source also of Old Saxon, Old Frisian, Middle Dutch, Old High German, German warm, Old Norse varmr, Gothic warmjan "to warm").

A word of uncertain origin. On one guess it is from PIE root *gwher- (source of Greek thermos "warm;" Latin formus "warm," Old English bærnan "to kindle"). On another it is connected to the source of Old Church Slavonic goriti "to burn," varŭ "heat," variti "to cook, boil;" and Lithuanian vérdu, virti "to seethe."

The use of distinct words, based on degree of heat, for warm and hot is general in Balto-Slavic and Germanic, but in other languages one word often covers both (Greek thermos; Latin calidus, French chaud, Spanish caliente).

In reference to feelings, etc., "ardent, full of zeal," it is attested from late 15c. Of colors from 1764. The sense in guessing games of "close to the thing that is sought" is recorded by 1860, probably from earlier use in hunting in reference to the scent or trail (1713).

Warm-blooded in reference to mammals is recorded from 1793. Warm-hearted, "of a generous and affectionate nature, inclined to seek friendship," is by c. 1500.

1540s, from cool on the model of warmth. It persists, and was used by Pound, Kipling, etc., but it never has shaken its odor of facetiousness and become standard.

suffix forming nouns of action, state, or quality from verbs or adjectives (such as birth, bath, depth, death, growth, strength, truth, math (n.2)), from Old English -ðu, -ð, from Proto-Germanic *-itho (cognates: Old Norse -þ, Old High German -ida, Gothic -iþa), abstract noun suffix, from PIE *-ita (cognates: Sanskrit -tati-; Greek -tet-; Latin -tati-, as in libertatem "liberty" from liber "free"). Sometimes in English reduced to -t, especially after -h- (as in height).

Formerly more widespread (Middle English had stilþe "silence," c. 1200; wrengthe "wrongness, crookedness, distortion," c. 1300), and in recent centuries often tempting to new coinages (17c. swelth "swelling;" Ruskin's illth).