(n.) a bell tower
Belfries have nothing to do with bells. Well, almost nothing.
Belfry is an ancient word. Evidence of similar terms appears in such a vast number of north European languages that it’s thought the word existed in some form in Proto-Germanic, one of the daughter languages (or perhaps granddaughter languages) of Proto-Indo-European that emerged as a language in its own right in northwest Europe sometime in the mid first millennium BC. It’s thought back then the word would have resembled something along the lines of bergfrithu—a compound essentially meaning ‘protected area of high ground’.
That berg is a fairly fruitful Germanic root that crops up in all manner of words from bourgeois to burglary. But as a word for an area of high protected land, it proved particularly useful in naming places: it’s this that you’ll find at the end of the likes of Edinburgh, Middlesborough, Strasbourg, Pittsburgh and Hamburg. But locked in initial position in belfry, something peculiar happened.
In the sense of something tall and related to military protection, a belfry was originally a mobile siege tower—essentially, a large timber column that could be wheeled up to the wall of a castle or city to attack it. From there, the word came to be used of any similarly tall, tower-like structure, and it is in that sense that it came to be used of the towers housing the bells of churches in the 1400s.
So ancient a word as this one, however, is always going to be susceptible to change, and over time that initial berg came to be more closely allied with the bells these towers contained. That L steadily (and mistakenly) came to replace the RG in berg, and the word belfry as it is today was born—along with the equally misguided notion that it was intentionally named as a bell tower.