When you don't have enough time, or don't care enough, to use a hairdryer. Thus, you put all the windows down in your car and let the wind do all the work.
(Not to be confused with a hippie hairdryer, which involves the same concept, but includes the use of a bike instead. This version is "greener",from which is derived its condition of being "hippie".)
Works best for guys.
Or girls with short hair.
"Hey, Becky sure has some serious bed head."
"Nope. Hobo hairdryer."