Rabbits!

"Rabbits!" was a negative exclamation favoured by Tegan Jovanka. She frequently used it to indicate displeasure or frustration at an event. (TV: Logopolis, Frontios; PROSE: Graham Dilley Saves the World, The Assassin's Story; AUDIO: Psychodrome, The Star Men, Aquitaine, The Peterloo Massacre, The Emerald Tiger, Eldrad Must Die!, The Entropy Plague)

Byron was also a frequent user of the exclamation. (AUDIO: The Girl Who Never Was)

Behind the scenes
The exclamation does not seem to particularly be popular, real world slang. It may have simply been a Janet Fielding-ism or an invention of the writers.

On an interview for Doctor Who Magazine 431, Janet explained: "Well, [Tegan] was hardly going to say, 'F*** off and die,' was she? Not on BBC1!"".