A
term used to
describe those
very very thin people who can eat anything they want and still remain
very very thin. Much like the
hummingbird, which eats 2/3rds of it's
body weight in
sugar every day, and stays, well, the size of a
hummingbird.
You can often tell when somebody has this metabolism by the way you can count their ribs through their shirt. Another good indicator is if they wolf down ice cream, soda pop, etc., and never gain any weight. Or even worry about gaining weight (this characteristic is more than a little bit irritating).
The other end of the spectrum, I suppose, must be the metabolism of a three-toed sloth.