Limpness is the noun: the state of being limp — that no-spring, not-firm quality. After a long walk, the limpness in your legs is the heavy feeling with no bounce left. Hair can have limpness on a wet day; it will not sit up. A handshake can have limpness too, when there is no life in it. It is a straightforward word, a little more formal than just saying “it’s limp,” but people will still understand it at once. It belongs with limp and limply, not with limpid.