Why is the ninth month called September?
Old English blædre (West Saxon), bledre (Anglian) "(urinary) bladder," also "blister, pimple," from Proto-Germanic *blaedron (cf. Old Norse blaðra, Old Saxon bladara, Old High German blattara, German Blatter, Dutch blaar), from PIE *bhle- "to blow" (see blast). Extended senses from early 13c. from animal bladders used for buoyancy, storage, etc.
bladder blad·der (blād'ər)
Any of various distensible membranous sacs, such as the urinary bladder, that serve as receptacles for fluid or gas.
A blister, pustule, or cyst filled with fluid or air; vesicle.