What is the X in X-mas?
fem. proper name, from French Mathilde, of Germanic origin, literally "mighty in battle;" cf. Old High German Mahthilda, from mahti "might, power" + hildi "battle," from Proto-Germanic *hildiz "battle," from PIE *kel- (1) "to strike, cut." The name also was late 19c. Australian slang for "a traveller's bundle or swag," hence the expression waltzing Matilda "to travel on foot" (by 1889).
In my electorate nearly every man you meet who is not "waltzing Matilda" rides a bicycle. ["Parliamentary Debates," Australia, 1907]The lyrics of the song of that name, sometimes called the unofficial Australian national anthem, are said to date to 1893.