What does calling someone a cow mean in Britain?

Posted by ___o----@reddit | AskABrit | View on Reddit | 286 comments

I’ve seen this insult a number of times in movies or tv shows and I’m always a little puzzled. It’s clearly derogatory, but if we called someone a cow in the US, we’d mean she’s really fat. In the UK, it doesn’t seem to mean that at all. Every woman referred to as a cow has been slender. So what’s the deal? Thx.