The word "cornet" is a diminutive that has outgrown its parent: from Latin cornu ("horn") through Old French cornet ("little horn"), it has come to name a brass instrument, an ice cream cone, a military rank, a type of headdress, and various other horn-shaped objects. The "little horn" turns out to contain a surprising amount.
Proto-Indo-European *ḱer- meant "horn" or "head" — the two concepts connected because horns grow from heads. This root generated one of the largest word families in the Indo-European languages. Latin cornu ("horn, antler") is the direct ancestor of an enormous English vocabulary: "horn" itself (via Germanic), "corner" (where walls meet at a horn-like angle), "unicorn" (one horn), "cornucopia" (horn of plenty), "cornea" (the horn-like transparent covering of the eye), "cornice" (a projecting horizontal molding), "Capricorn" (goat horn), and "coroner" (originally a Crown officer — corona from cornu).
Old French formed cornet as a diminutive of corn ("horn"), meaning "little horn" or "horn-shaped object." English borrowed this in the 14th century, initially for various horn-shaped items: a small horn used for signaling, a horn-shaped container, a conical paper wrapping for sweets or other small goods.
The musical instrument that modern English speakers know as a cornet developed in the 1820s from the post horn — a small, valveless horn used by postilions on mail coaches. The addition of valves (initially two, later three) created an instrument capable of playing chromatic scales, and the cornet quickly became one of the most popular instruments in 19th-century music. It is closely related to the trumpet but has a more conical bore, giving it a warmer, mellower tone.
The cornet dominated popular and band music throughout the 19th and early 20th centuries. In American music, the cornet was the lead instrument in early jazz: Buddy Bolden, King Oliver, and the young Louis Armstrong all began as cornetists before the trumpet gradually displaced the cornet in jazz ensembles. The brass band tradition, strong in both Britain and America, gave the cornet a central role that it retains in British brass bands to this day.
The ice cream sense — a cornet as a cone-shaped wafer holding a scoop of ice cream — preserves the word's original meaning most directly. A cornet is simply a little horn, and a wafer cone is a horn-shaped container. This usage is standard in British English ("an ice cream cornet") while American English prefers "cone." The Italian cornetto, a cousin word, refers both to a croissant (horn-shaped pastry) and to the Renaissance wind instrument — different horn-shaped objects claiming
The military cornet was originally a standard-bearer — the officer who carried the regiment's cornet (pennant, itself a diminutive horn-shaped flag) in cavalry units. The rank survived in the British Army as the most junior commissioned rank in cavalry and armored regiments until it was replaced by "second lieutenant" in the 19th century.
From a Proto-Indo-European horn through Latin and French, the "little horn" has proved endlessly adaptable — naming anything that curves, points, contains, or sounds like a horn. The word is a testament to the metaphorical fertility of basic shapes: give a language a horn, and it will find a hundred things to call by its name.