What is the X in X-mas?
c.1400, "A wind instrument made of wood and provided with six finger holes" [Middle English Dictionary], from Old French cornet (14c.) "a small horn," diminutive of corn "a horn," from Latin cornu "horn" (see horn (n.)). Modern use is short for cornet-à-pistons "cornet with pistons."
The quality of the tone is penetrating and unsympathetic, by no means equal to that of the trumpet, for which it is commonly substituted. ["cornet" entry in "Century Dictionary and Cyclopedia," 1902]
Heb. shophar, "brightness," with reference to the clearness of its sound (1 Chr. 15:28; 2 Chr. 15:14; Ps. 98:6; Hos. 5:8). It is usually rendered in the Authorized Version "trumpet." It denotes the long and straight horn, about eighteen inches long. The words of Joel, "Blow the trumpet," literally, "Sound the cornet," refer to the festival which was the preparation for the day of Atonement. In Dan. 3:5, 7, 10, 15, the word (keren) so rendered is a curved horn. The word "cornet" in 2 Sam. 6:5 (Heb. mena'an'im, occurring only here) was some kind of instrument played by being shaken like the Egyptian sistrum, consisting of rings or bells hung loosely on iron rods.