"a helmet, a defensive cover for the head," from Old English helm "protection, covering; crown, helmet," from Proto-Germanic *helmaz "protective covering" (Cognates: Old Frisian, Old Saxon, Old High German helm, German Helm, Old Norse hjalmr, Gothic hilms), from PIE root *kel- (1) "to cover, conceal, save." Italian elmo, Spanish yelmo are from Germanic.
"loose white robe," c. 1200, from Old French surpeliz (12c.), from Medieval Latin superpellicium (vestmentum) "a surplice," literally "an over fur (garment)," from Latin super "over" (see super-) + Medieval Latin pellicium "fur garment, tunic of skins," from Latin pellis "skin" (from PIE root *pel- (3) "skin, hide"). So called because it was donned over fur garments worn by clergymen for warmth in unheated medieval churches.
In 18c., gown was the common word for what now usually is styled a dress. It was maintained more in the U.S. than in Britain, but was somewhat revived 20c. in fashion senses and in combinations (such as bridal gown, nightgown). Meaning "flowing robe worn on official occasions as a badge of office or authority" is from late 14c. As collective singular for "residents of a university" (1650s) it typically is used in rhyming opposition to town.