|If a shear stress is applied to the surface of a solid, the solid will deform a little, and then remain at rest (in its new distorted shape). One can say that the solid (at rest) is able to resist the shear stress.|
Now consider a fluid (in a container). When a shear stress is applied to the surface
of the fluid, the fluid will continuously deform, i.e. it will set up some kind of
flow pattern inside the container.
In other words, one can say that the fluid (at rest) is unable to resist
the shear stress. That is to say, it cannot remain at rest under application of a shear stress.