The word 'spittle' has a direct and ancient lineage within the Germanic language family. It originates from Old English 'spātl' or 'spǣtl', a noun formed from the base of the verb 'spǣtan', meaning 'to spit'. This verb itself can be traced back to the Proto-Germanic 'spait-', which is likely onomatopoeic in origin, imitating the sound of spitting. The modern word retains the core, concrete meaning of its ancestor, specifically denoting saliva, particularly when ejected from the mouth.

The word is a simplex form derived from the Old English noun 'spātl'/'spǣtl', which is related to the verb 'spǣtan' (to spit). It is not productively segmented into distinct morphemes in modern English.