E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A military tower, pushed by besiegers against the wall of a besieged city, that missiles may be thrown more easily against the defenders. Probably a church steeple is called a belfry from its resemblance to these towers, and not because bells are hung in it. (French, beffroi, a watch-tower, Old French, berfreit, belefreit, from German, berg-frit, bergen, to protect, frit [vride], a place fenced in for security.)