Both, so it depends on the context you use it. - Mi perra está enferma y la he llevado al veterinario - Carlos es un hijo de perra. No quiero volver a verlo más Anyway, in Spain we use the word 'zorra' (lit. 'vixen') as equivalent to 'bitch': - Kim Kardashian es una zorra que vive de los medios y de su cuerpo

It means female dog, we don't use it much as an insult, just in some cases but I won't write them here :)

Guillermo Cruz, gracias! :)

Both. But its an insult when you reffer to a women. Use a better dictionary like rae: http://lema.rae.es/drae/srv/search?id=ixr1f53DRDXX2DnmfGfD