(n.) the grease used to lubricate church bells
The grease that lubricates church bells is called bellcoom. And if you’re wondering why we would ever require a separate word for precisely that, then here’s something extra for you: it was once considered a cure for shingles.
Quite where this bizarre medical application of bellcoom comes from is anyone’s guess. Where the word itself comes from, though, is more explainable.
Coom is an old word for dirt or soot. It’s one of a number of similar words—including culm, coam, collow and colly—that have been used for smoky grime in English since the Middle Ages. Although their origins are unclear, it’s likely they’re all in some way descended from coal.
The sooty dust in coom was formerly mixed with grease and used as a lubricant for axles, shafts and, it appears, bells. This use of the word dates back to the early 1700s at least, but the practice itself is likely much older.
Precisely what the sooty coom mixed into the grease is meant to do is unclear, but it is this blackened mixture of dust and lubricating sludge that appears in the word bellcoom.