Why does my cat sometimes run at me sideways?
I've seen my cat charge at me from a distance while running sideways. He simply stops when he reaches me. I've read this is called "sidewinding" and some say it's to look intimidating, others say it's just being playful, and perhaps it's both, which I suspect is the case--do they do that in the wild with real opponents to intimidate them? I think he's just playing, but I'm curious whether it's an instinctual defense mechanism in an untamed setting.
It is an attempt to look large and threatening and drive you off. In the wild, a cornered cat might do it, or a mother protecting kittens. Usually a cat will slink off and hide, though.
And when playing, a kitty might do it to be playful? Or is it always aggressive, and in this case, temporarily angry about something? It makes sense to me that they'd do it in the wild when fighting, but I'm just trying to understand it in the context of playing when no offensive measures are called for.
Yes, a cat can during play act out the same acts that it uses in the wild in all seriousness. So in the home, it would be like saying "Look how big and scary I am! Yarr!!". Cats play hunt, play stalk, and play fight, and play kill with us and each other all the time.
Play is often practice for more serious adult activities. Kittens play fight and play hunt, and then as adults they use the tactics that worked for them when they practiced during play. Exaggerated body language is a signal to telegraph to play partners to that the kitten is playing and not really seriously attacking. It's sort of like if I pretend (in play) to hit someone, I might use a really big wind-up for my play punch so they can easily see it coming and can easily avoid it.