Dirt is defined as any unclean substance, such as mud, dust, excrement, etc; filth. However, I am not sure if dried pee is considered dirt, because it's not visible and it's barely solid. You can't scrub it off and it's soluble. So is it considered dirt or not? I know this question is weird and sound stupid, but the answer isn't as simple as it might appear.
For example:
He went to the bathroom to pee a while ago, so he took a bath to clean off the dirt on his private parts.