What is the X in X-mas?
"characteristic quality of a musical sound," 1849, from French timbre "quality of a sound," earlier "sound of a bell," from Old French, "bell without a clapper," originally "drum," probably via Medieval Greek *timbanon, from Greek tympanon "kettledrum" (see tympanum). Timbre was used in Old French (13c.) and Middle English (14c.) to render Latin tympanum in Ps. 150.