Rabbits!

"Rabbits!" was a negative exclamation favoured by Tegan Jovanka. She 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)

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!"".