A hairdryer was a device used to dry hair, usually after it was washed.
As the Ninth Doctor searched through a stockpile of weapons in Adam Mitchell's workshop in GeoComTex to destroy the last of the Daleks, he dismissed one of several weapons as a "hairdryer." (TV: Dalek) It was later salvaged and sold at auction as a "heat ray disintegration device" for 80 million. (COMIC: The Fountains of Forever)
Hairdryers could interfere with the sonic screwdriver. (TV: Forest of the Dead)