1520s, from Spanish or Portuguese don, title of respect, from Latin dominus "lord, master." The university sense is c.1660, originally student slang; underworld sense is 1952, from Italian don, from Late Latin domnus, from Latin dominus (see domain). The fem. form is Dona (Spanish/Portuguese), Donna (Italian).
early 14c. contraction of do on (see doff). "After 1650 retained in popular use only in north. dialect; as a literary archaism it has become very frequent in 19th c." [OED]. Related: Donned; donning.