The first time I saw this word, context told me it was referring to a celestial being, so I assumed that it was some magnificent angel with a harp. Until it became clear that it was not. At all. It didn’t take long. The realization was like that scene at the end of Raiders of the Lost Ark.
A harpy does not carry a harp. And, in the original, it does not harp on about things. It just shows up and deals horrible punishment. This is the Oxford English Dictionary definition: “A fabulous monster, rapacious and filthy, having a woman’s face and body and a bird’s wings and claws, and supposed to act as a minister of divine vengeance.” More generally, as applied to people, it describes not someone acting as an agent of righteous retribution but simply a vicious, predatory person.
Or, in commonest use, a belligerent, captious, verbally abrasive woman. The kind who won’t stop harping on about things. Often, in fiction, an ex-wife, or an ex-girlfriend, or a mother-in-law, or someone’s sister or aunt.
Yes, a common usage of this word is inescapably misogynistic. The original beast had a woman’s face, so you may say that’s just how it is, but do stop to wonder why it didn’t have a man’s face, and whether it would have been given a different character if it did. What, in fact, is our equivalent word for a man? I mean, I don’t want to harp on about this, but…
Oh, and while the ‘go on and on about something’ sense of harp has quite evidently helped shape the current usage of this word, the instrument harp and the beast harpy really are not related. The musical instrument’s name comes from an old Germanic root. The nasty celestial being, on the other hand, comes to us by way of Latin harpyia, usually seen in the plural harpyiæ; that is taken (as the y strongly hints) from Greek, in this case ἅρπῡιαι harpuiai ‘snatchers’, which have actually been different things in different myths: personifications of winds and storms in Homer, goddesses in Hesiod, and later on the winged creatures, which carried people and things off, such as the souls of the dead to Hades.
Snatchers? That sounds rapacious. Could we imagine the harpy is a raptor? We could – in particular if we refer to the harpy eagle, which is an eagle of the Americas, larger than the golden eagle. It’s named after the mythical beast, reasonably enough. Your soul may be safe around it, but your cat probably isn’t.