Dirty is also a reference to an aircraft configuration. When an aircraft is “dirty”, that means it is in landing configuration, flaps and landing gear down. Performance numbers for complex aircraft often will include clean and dirty configuration information, the most important of these being stall speed. This is usually considerably slower than an airplane in clean configuration, as there is more drag. The extra drag is being introduced by the extended landing gear, as well as the extra drag associated with the added lift that the flaps provide.