The phrase "thick mud" refers to mud that is very gooey, sticky, and dense. It is difficult to walk through or move in because it is heavy and covers your shoes or tires. Full definition
Its tires are coated in several inches of thick mud, which caused the rear fender liners to pop out. (automobilemag.com)
Combined with almond meal, which is naturally higher in oil content than a regular flour, you get that nice thick mud cake texture. (nourisheveryday.com)
Some hard walking through thick mud paid off when it was reached, revealing another fossil methane seep. (sciencedaily.com)