I understand "dirty" software as non-GPL stuff, stuff the very honourable Free Software Foundation does not favour.
On the other hand, dirty software can be something else, horribly dirty:
Dirty proprietary software needing to be regularly validated, registered, allowing "Big B." to snoop out the contents of your hard drive erasing what He doesn't like at His own will without even asking you, deactivating your VERY expensive legal copy of your paid-for software, deleting your personal stuff. What would you say if if your operating system vendor secretly erased you CV without telling you?
I think we should all read the license conditions instead of just clicking them away!