Oh yes. It's insulting. You're saying "I am kicking your ass" which means "I am beating you". Saying "My foot is in..." is a way of saying it I hadn't heard, but I understand the meaning. I also get a sense that this emphasizes the idea that "I am way ahead of you, you are slow in catching up" I juess because it implies that I have already kicked you and you haven't even noticed it yet. When can you use it? When you are yelling at someone? When you are taunting your opponent in pro wrestling?

there are no exact meanings to these phrases in the way you want. but a lot of people say things like "don't make me break my foot off in your ass" basically it means im gonna kick your ass SO HARD that my foot will go inside of it. and then i cant get it out, and it will break off. as for kicking some ones ass, is slang for Beating them up. or performing karate on them etc...

I would agree. Not only is it insulting and unkind, it's rather coarse. I wouldn't use it either. It doesn't make you look good.

But I suggest you not to use it :))

Don't use it!!!!