What is the X in X-mas?
1900, but perhaps as early as 1885, involuntary transposition of sounds in two or more words (cf. "a well-boiled icicle" for "a well-oiled bicycle;" "scoop of boy trouts" for "troop of Boy Scouts"), in reference to the Rev. William A. Spooner (1844-1930), warden of New College, Oxford, who was famous for such mistakes.
A reversal of sounds in two words, with humorous effect. Spoonerisms were named after William Spooner, an English clergyman and scholar of the late nineteenth and early twentieth centuries. In one spoonerism attributed to him, he meant “May I show you to another seat?” but said, “May I sew you to another sheet?”