A shameless flirt is someone who will flirt with anyone. They do not even necessarily need to be attracted to them. These types of people flirt constantly, it is their default setting. When they are flirting with you, you know it. There is no question about it. Don't feel too special though, shameless flirts do it because it is fun and comes naturally to them, not because they particularly like the person.
Jordan: That girl was flirting with me, I think she likes me.
Matt: Nah, she is such a shameless flirt. Don't feel too special.