Cruk was originally a primitive method of constructing a house, using a material similar to dry-stone wall or wattle and daub. The term became associated with the extreme poverty of some of the newer Earth colonies before linguistically metamorphosing into a term of abuse before its final appropriation as a popular human expletive, used amongst the inhabitants of the Terran Federation in the twenty-sixth century as a means of expressing their displeasure (AUDIO: Death and the Daleks). The Doctor's companion Bernice Summerfield considered it a favourite word of hers and the staff of the Pandora Spindle were rather fond of applying the term profusely during a moment of crisis (PROSE: Love and War, et al.; COMIC: Pureblood).
Behind the scenes
Like Battlestar Galactica's frak, the word was a "space age" equivalent of fuck, which could be used in exactly the same situations as the modern-day swearword without offending members of Virgin's target audience. The term was first invented by Paul Cornell in Benny's debut novel Love and War and has remained associated with the character ever since.