The word 'unsparing' is built from the core verb 'spare,' which originates from the Old English sparian, meaning "to refrain from, to be lenient to, to leave undamaged." This concept of holding back, whether from destruction or from use, is central. Adding the present participle suffix '-ing' creates an adjective meaning "not holding back." The negative prefix 'un-' intensifies this to mean "not at all holding back." This dual logic explains the word's two modern senses: in a negative context, it means not holding back severity or criticism; in a positive context, it means not holding back one's generosity or effort.

Structure: un (not) + spare (refrain from using/destroying) + ing (adjective suffix) - un: A prefix of Germanic origin (Old English) meaning "not," used to form negatives. - spare: A verb of Germanic origin (Old English sparian) meaning to refrain from harming, punishing, or using. - ing: A suffix of Germanic origin (Old English -ende, -inge) used to form present participles and adjectives.