A thin layer of facial hair on the upper lip that looks like dirt was smeared over the persons face.
Look at AJs dirtstache, did he dive into a pile of shit?
(n) A five o'clock shadow above the upper lip that looks like a line of dirt, rather than a moustache. Usually seen on greasy adolescent boys. Not uncommon in women.
Joe shaved and shaved his upper lip so he could fit in and have a moustache like Pedro. Well indeed Joe was not as hairy as Pedro, and developed only a dirt stache instead.
n. A layer of dirt on the upper lip and nose from riding dirt bikes/ATVs in a dusty region, particularly where the helmet does not cover the face.
I was riding behind you for miles, all that dust gave me the worst dirtstache!