The word is a transparent English compound formed from two native Old English elements. The prefix 'under-' directly denotes the positional sense of being worn beneath outer clothing. The plural noun 'things' serves as a vague, collective term for items or objects, a common euphemistic device in English for personal or intimate articles. This combination, 'underthings', emerged as a modest, somewhat old-fashioned term to refer to undergarments without specifying their type, paralleling the formation of similar terms like 'underclothes'. Its logic is entirely literal, describing "those things which are worn underneath."

Structure: under (beneath) + things (objects) under: Old English origin, meaning "beneath, below, among." It functions as a prefix indicating position or location. things: Old English origin, from 'þing', meaning "an object, entity, matter, or affair." It functions as a noun-forming element, here used collectively.