Slang term used in erotic contexts, especially in BL (boys' love) and adult media, for the character who takes the active, penetrative role. Derived from 竿 (さお, 'rod') + 役 (やく, 'role').
Compound of 竿 (さお, 'rod, pole') and 役 (やく, 'role, part'). The term likely originated in erotic manga and doujin circles as a euphemistic way to refer to the penetrative partner.