Descriptive words are used in order to help readers create mental images of what is being described. This is typically referred to as imagery. The best images appeal to all five sense of the reader. When all five senses are appealed to, the reader is able to create a mental picture with depth.
Therefore, descriptive words which can appeal to the senses of a reader (in order to create the most vivid image) are as follows.
Visual- Grungy, filthy, ragged, ape-like,
Smell- Earthy, musky, sweaty
Sound- Gnashing teeth, clubbing the earth, breaking bone, monosyllabic, guttural
Touch- Matted hair, course fur covered clothing, grit-covered
Taste- Looking at him made my mouth feel like I was chewing on sand.
Assuming by caveman you want to create an image of the stereotypical primitive, here are some descriptions:
grunting (when describing the cave-man's speech)
You can also describe the forehead or brow as being prominent, or go into more details about the "fur-clad" aspect by saying it was a fur loincloth, specifically. You might also say the hand that wasn't holding the club was dragging along the ground by his side, giving him an ape-like appearance.
If you aren't going for the stereotypical caveman image, then you can just go crazy with whatever descriptions you wish!