The term ‘flying dirty’ refers to when an aircraft is flying with all of the slats, flaps, and wheels hanging down. This is opposed to a clean configuration in which everything is where it is meant to be and hidden away.
It is not common for pilots to fly dirty as this is not how planes are meant to be used. It is often unheard of when it comes to commercial aircraft.
Another potential meaning to the term ‘flying dirty’ could mean to fly upside down. Some aircraft do fly dirty in airshow formations. For example, the Blue Angels flew dirty in their 40th-anniversary ceremony to unveil a new aircraft called the McDonnell Douglas F/A-18 Hornet.
On 8th November 1986, the Hornet flew dirty with the landing gear down for a formation loop. This sure impressed viewers and amped up the popularity of ‘flying dirty’, or flying upside down, in many air shows.
Don’t worry, though, your pilot of a commercial aircraft is not about to take you upside down. At least, not without telling you first!