When money is considered to be dirty money it essentially means that any money that was illegally obtained such as through a robbery, burglary, forgery, bribery, prostitution, money laundering and just theft of money.
Sometimes it's hard to know whether the money you have gotten from someone is dirty money or stolen money because you might get paid for something by someone and not know they stole or got the money through illegal ways.
I'm sure money we've held and spent has been gotten from someone else illegally and then passed onto us through stores, banks etc without us or them knowing.
The only way you'd really know if money was actually dirty money would be if you or someone you know actually got the money illegally.
But you may not know if you have money that is stolen or considered illegal.
Sometimes you might get some money that is not real and counterfeit through no fault of your own and just because you might try to use it doesn't mean you're gonna get into trouble.
That is as long as you didn't actually counterfeit the money yourself.