Steamy is most often used as an adjective to describe the appearance of an individual. It can be a substitute word for 'hot', 'sexy', etc.
An individual who is steamy is often highly desirable, very good-looking/attractive, and normally has a great body. They also bring a 'steaminess' with them when they enter a room.
Being called steamy is often better than any other compliment one could receive (ie. hot, sexy, attractive, cute) Steamy people are a very rare breed. Being called steamy is something to be very proud and flattered about.
He is so incredibly steamy, I've never seen anyone hotter than him or with a better bod than him in my life.