Nag: noun - a pony or small horse suitable for riding; colloquially, any horse; the penis; an act of nagging; something or someone that nags; a person who nags persistently, especially a woman.
'You've never put money on Glorious Failure to win, have you? That old nag won't even make the first jump.'
'You know what you are, don't you? You're nothing but a nag: nag, nag, nag, all the time. You'll be the death of me with your eternal carping, you evil little nag.'
Pic: The waterfall at Goredale Scar, North Yorkshire.