If the Poisson's ratio of a material is 0.5, then the material is:

Bulk modulus $K$ is related to Young's modulus $Y$ and Poisson's ratio $\sigma$ by $K = \frac{Y}{3(1-2\sigma)}$. If $\sigma = 0.5$, then $(1 - 2\sigma) = 0$, making $K \to \infty$. An infinite bulk modulus means the material cannot be compressed, i.e., it is perfectly incompressible.