Clammy means unpleasantly cool and slimy to the touch. It has nothing to do with those tasty little sea creatures, but they too are pretty slimy.
Nine times out of ten the thing described as clammy is a hand or forehead, usually of someone who's pretty sick or just physically unpleasant. When Dickens' David Copperfield shakes the hand of the creepy Uriah Heep he exclaims, "But oh, what a clammy hand his was! As ghostly to the touch as to the sight!" Clammy can also refer to the air or atmosphere of a place, if it's particularly damp.