A stupid person; it refers to the lack of surface area on an individual's brain. The general thought is that the more surface area (wrinkles, creases, etc.) a brain has, the smarter the person is. Conversely, a person with a smooth brain (no wrinkles) has less surface area and would therefore be stupid.
That fucking smooth brain put his shirt on backwards again...
(1) A cad, rogue or bounder. A highly-functioning, charismatic sociopath who has a purely instrumental view of others. Treats women like objects and vice versa.
(2) A medium-sized shark in the genus Mustelae.
(1) "You cheated on your Cambridge finals, sold crack to your aunt, gave the hiv and the herp to many student nurses, and later became the chairman of an investment bank. What kind of man are you, for God's sake?"
"I'm a smooth-hound. That's how we roll."
(2) "The smooth-hound is a very ordinary shark; its name is the most interesting thing about it. Smooth-hound."