Its source is a Latin expression meaning "One."
This girl's name is used in Scots Gaelic and Irish Gaelic.
Elizabethan poet Edmund Spenser (1552-1599) used the name Una for the heroine of Book I of his allegorical poem The Faerie Queene. Spenser may have heard an Irish name meaning either ''hunger'' or ''lamb'' that sounds the same as this name, but it is more likely that he had the Latin term meaning ''one'' in mind. His Una symbolized the one, true (i.e., Anglican) church.
Spenser contrasted Una with another character who falsely claimed to be named Fidessa (''faithful''), but who in fact was named Duessa (''two-faced''). This femme fatale tried to win George, the Redcross Knight (and symbol of England) away from the virtuous Una, and so symbolized the Roman Catholic Church striving to lure England back into its fold.