the.com/tail fin

means The upright fin at the rear of a vehicle (classically a car) that helps with stability and, at least historically, looked fast whether or not it did anything useful.

from The phrase simply fuses 'tail' (the rear end of a thing, from Old English tægl) with 'fin' (an animal's swimming appendage, from Old English finn, related to words across Germanic languages for the same fish part). Engineers and stylists borrowed the fish's fin shape wholesale — literally, for aircraft rudders and stabilizers, then flamboyantly, for 1950s car bodies inspired by jet planes, where the fin's aerodynamic job mattered far less than its silhouette.