Tailfin is a compound of tail (Old English 'hind part of an animal') + fin (Old English 'fin of a fish'), forming Modern English tailfin, a decorative fin-shaped extension on 1950s-style cars. In current use it is a noun meaning "a decorative upswept extension on the rear of a car, popular in the 1950s and 1960s". That sense is explained by the older material without requiring a speculative semantic step: the inherited or borrowed base supplies the field of meaning, while the English formation fixes its modern role. The surface form is compositional: its joined elements remain visible in the modern word. The semantic development is therefore important as well as the form: the word moves from the older concrete, technical, or literal setting into the present figurative or specialized sense.