  We argue that the Kovtun--Son--Starinets (KSS) lower bound on the viscosity
to entropy density ratio holds in fluid systems but is violated in solid
materials with a non-zero shear elastic modulus. We construct explicit examples
of this by applying the standard gauge/gravity duality methods to massive
gravity and show that the KSS bound is clearly violated in black brane
solutions whenever the massive gravity theories are of solid type. We argue
that the physical reason for the bound violation relies on the viscoelastic
nature of the mechanical response in these materials. We speculate on whether
any real-world materials can violate the bound and discuss a possible
generalization of the bound that involves the ratio of the shear elastic
modulus to the pressure.
