From the old Irish 'Eidhne' pronounced 'Ay-nah'. A term from the west of Ireland meaning one of unscrupulous intentions towards one fo the opposite sex. Its origins are not fully know as it deviates from its original irish meaning of 'little bird'.
"That Forde lad is some eanna on the dancefloor."
"She's some slapper, she goes all eanna as soon as any lad starts talkin' to her"