Thon. Four letters, one syllable, and it's just "that one" with the middle filed out.
Question I've used in a round twice: which pronoun did an American lawyer invent for English, and how long did a dictionary keep it? Thon, and about seventy years — Funk & Wagnalls had it in the Standard Dictionary from the end of the 1890s until the 1960s, straight-faced, edition after edition. (There was a possessive, thons. And a reflexive, thonself, which I have never once said out loud without grinning.) Charles Crozat Converse was born in 1832, floated it in the 1850s, pushed it again in the 1880s, and died in 1918 with roughly nobody using it.
The other thing he made has done rather better. He wrote the tune to "What a Friend We Have in Jesus" — the one you've sung at a funeral whether you meant to or not, in a church you were visiting, from a hymn book someone handed you. (I can never get the second verse without reading it.) The words to that weren't his. Joseph Scriven wrote them as a poem to comfort his mother, away in Ireland, and Converse set them to music years afterwards.
Scriven sent those words to his mother in Ireland as a private consolation, and they went into print anonymously at first — so for a while the hymn had a tune with a name on it and words with none. He was, as far as I know, rather surprised to find himself its author in public.
Webster's Second carried thon as well, so it had two dictionaries vouching for it and still no speakers. Meanwhile singular they had been quietly doing the job since Chaucer's century, unregistered, the way most useful words get in — nobody invented it, so nobody had to campaign for it.