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.).
Proto-Indo-European root meaning "high," with derivatives referring to hills and hill-forts.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit b'rhant "high," brmhati "strengthens, elevates;" Avestan brzant- "high," Old Persian bard- "be high;" Greek Pergamos, name of the citadel of Troy; Old Church Slavonic bregu "mountain, height;" Old Irish brigh "mountain;" Welsh bera "stack, pyramid."
also bat-shit, by 1967 as a variant of bullshit (n.) in the slang sense; from bat (n.2) + shit (n.). By early 1980s as "crazy," the sense shift is for uncertain reasons; perhaps (these are very long shots) from the notion of guano as an explosive or health problems caused by inhaling powdered bat feces in caves and mines. Also compare batty "crazy" (early 20c.), from the expression bats in (one's) belfry.