This can either refer to blood that just clots more easily than normal, or blood that is actually physically thicker than normal.
Blood can clot easier if a person has a genetically inherited disorder that makes their normal clotting mechanism hyperactive. There are also medications such as birth control pills which increase clotting.
Blood can be physically thicker as well, such as when there are high levels of sugars (like in poorly controlled diabetes
) or triglycerides (such as in people with elevated cholesterol
). It can also be thicker if there are too many red blood cells. This can happen in certain genetically inherited blood disorders, or if a person is using testosterone
supplements or EPO to boost their sports performance (that's why they test athletes for their concentration of red blood cells).