rail-thin

The adjective "rail-thin" is a vivid 20th-century American compound. Its first element, "rail," draws on the concrete image of a slender, rigid metal bar used in fences or tracks, a meaning derived via Old French from the Latin regula, meaning a straight stick or ruler. The second element, "thin," is a fundamental Germanic adjective describing lack of thickness. The compound's logic is purely metaphorical, comparing a person's extreme slenderness to the narrow, linear form of a rail, creating an intensified meaning beyond simply "thin." This evolution showcases how everyday objects (rails) provide potent imagery for describing human physicality in modern English.

Structure: rail (rail) + thin (thin) - rail: From Old French reille (iron bar), from Latin regula (straight stick, rule). In this compound, it functions as a metaphorical noun meaning a slender bar or rod, used to intensify the following adjective. - thin: From Old English þynne (not thick, sparse), from Proto-Germanic thunnuz. It functions as the core adjective describing the state of having little flesh or fat.