Old English wearm "warm," 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"), 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 guess 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., attested from late 15c. Of colors from 1764. Sense in guessing games first recorded 1860, from earlier hunting use in reference to scent or trail (1713). Warm-blooded in reference to mammals is recorded from 1793. Warm-hearted first recorded c. 1500.
suffix forming nouns of action, state, or quality from verbs or adjectives (such as depth, strength, truth), 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).