Viscosity is a measure of the resistance of a fluid (a liquid or gas) to shear or deformation generated either by gravity or by an external force. It can be understood as the "toughness" of a fluid.
In terms of bulk sampling, the term "viscosity" is probably less relevant, as bulk solids are generally solids in loose form, such as sand, gravel, coal, grain, etc. comprises.
However, there are situations where viscosity could become relevant. If the bulk material is damp or wet, it may cause lumping or clumping, which can make sampling difficult. In such cases, the viscosity of the bulk material could play a role, as a higher viscosity material is harder to handle and sample.
Viscosity is usually measured with a viscometer. There are many different types of viscometers based on different physical principles. Some measure the force required to move an object through the fluid, others measure the time it takes to allow a certain amount of fluid to flow through an orifice, and so on.
Please note that when sampling high viscosity bulk material or wet bulk, special sampling and handling techniques may be required to ensure that the sample is representative and will not be damaged or altered.