belfry (n.)

c. 1400, "wooden siege tower on wheels" (late 13c. in Anglo-Latin with a sense "bell tower"), from Old North French berfroi "movable siege tower" (Modern French beffroi), from Middle High German bercfrit "protecting shelter," from Proto-Germanic compound *berg-frithu, literally "high place of security," or that which watches over peace." From bergen "to protect" (from PIE root *bhergh- (1) "to hide, protect") or [Watkins] *bergaz "mountain, high place" (from PIE root *bhergh- (2) "high," with derivatives referring to hills and hill-forts) + *frithu- "peace; personal security" (see affray).

The etymological meaning was forgotten, which led to folk-etymologies and a great diversity of spellings. It came to be used for bell towers (mid-15c.), which at first often were detached from church buildings (as the "Leaning Tower" of Pisa and the Campanile on Plaza San Marco in Venice), and the spelling was altered by dissimilation or by association with bell (n.).