early 14c., "close-fitting cap worn by women," from French cale "cap," back-formation from calotte, from Italian callotta, from Latin calautica "type of female headdress with pendent lappets," a foreign word of unknown origin.
The "cap" sense was the main one until 17c. Medical use, in reference to various membranes, dates to late 14c.; especially of the amnion enclosing the fetus before birth from 1540s. This, if a child was born draped in it, was supersititously supposed to betoken prosperity, give the gift of eloquence, and protect against drowning (18c. seamen paid dearly for one, and cauls were advertised for sale in British newspapers through World War I).