A second lover or gallant. It was anciently believed that twins could not be the genuine offspring of one man, a notion alluded to here.--James Halliwell's Dictionary of Archaic and Provincial Words, 1855
Feast Eve of Sts. Damian and Cosmas,
Arabian patrons of apothecaries. These twin brothers charged no fee for their services, and as a result became known as "the moneyless ones." A Celtic belief that made inroads in British and Northern European psychology suggested that the birth of twins was a result of the mother's sexual union with the devil after normal fertilization had occurred. In early times, the second born was not necessarily put to death directly, but rather abandoned by being put out into the elements without protection, to allow the evil spirits to reclaim their own. It was conjectured that legendary Sir Lancelot was just such a child, raised by fairies. Into the late nineteenth century, some parents believed a male fraternal twin had to be baptized before his sister or he might develop effeminacy, and the girl masculine traits.
Bah. The dark ages sucked!