the word 'underarm' is a transparent compound formed from two old english elements. 'under' derives from proto-germanic under, which has a stable meaning of "beneath," and can be traced back to the proto-indo-European root ndher-, also meaning "under." the element 'arm' comes from proto-germanic armaz, originating from the pie root h₂er- meaning "to fit together," which evolved to refer to the jointed limb. the compound logically and literally describes the area located beneath the joint of the upper limb, a meaning that has remained consistent from its formation in english to its modern use in both anatomical and descriptive contexts.

Structure: under (beneath) + arm (limb) 'under' is from old english 'under', meaning beneath or below, functioning as a prepositional prefix. 'arm' is from old english 'earm', meaning the upper limb of the body, functioning as the root noun.