Unmentionables

Within the Dalek City, described the Doctor's TARDIS to the Daleks as "the dog's unmentionables", doing so while tickling a nearby Dalek's sense globes. (TV: The Magician's Apprentice)

Behind the scenes
Michelle Gomez confirmed on Twitter that her tickling a Dalek's bumps when referring to the TARDIS as "the dog's unmentionables" was improvised.