The word 'underfed' is a transparent compound formed within Modern English from the native Germanic elements 'under' and 'fed'. The prefix 'under', with its core sense of "beneath," is used figuratively to denote a deficiency or falling short of a requirement. Attached to the past participle 'fed', it creates a participial adjective describing the state resulting from an action—in this case, the action of feeding done to an insufficient degree. This logical construction directly parallels other 'under-' formations like 'underpaid' or 'understaffed', efficiently conveying the concept of being below an adequate level of nourishment.

Structure: under (below, insufficiently) + fed (past participle of 'feed') - under: From Old English under, meaning "beneath, among, before, in the presence of." In this compound, it functions as a prefix indicating insufficiency or being below a standard. - fed: The past participle of the verb 'feed', from Old English fēdan ("to nourish, give food to"). In this adjective, it indicates the state of having been fed.