Anglicized spelling of Síle, the Irish Gaelic form of Cecily. This name has become so common and widespread that it is hardly felt to be Irish any longer. In Australia since the 19th century it has been a slang generic term for any woman.
Respelling of French Boillard, a pejorative nickname for a corpulent individual, from a derivative of Latin botulus ‘guts’, ‘belly’. In Burgundy boille also denotes a large basket for collecting grapes and it is possible that in this area the name arose as a metonymic occupational name for someone who made or used such baskets.