Latinate name, first used by Shakespeare for the rich heiress wooed by the duke in Twelfth Night (1599). Shakespeare may have taken it as a feminine form of Oliver or he may have derived it from Latin oliva ‘olive’. In the 1970s it was particularly associated with the Australian pop singer and actress Olivia Newton-John (b. 1948). Since the 1990s it has been very popular throughout the English-speaking world.
Irish (Ulster, Leinster, and Kerry): reduced Anglicized form of Gaelic Ó Leathlobhair ‘descendant of Leathlobhar’, a personal name composed of the elements leath ‘half’ (i.e. ‘somewhat’, ‘fairly’) and lobar ‘leprous’, ‘sick’. The name seems to have been originally a byname for a man of unhealthy constitution.