Scrapie
The name scrapie is derived from one of the clinical signs of the condition, wherein affected animals will compulsively scrape off their fleeces against rocks, trees, or fences. The disease apparently causes an itching sensation in the animals. Other clinical signs include excessive lip smacking, altered gaits, and convulsive collapse.
Crab (scratch)
While the name "crab" seems self-explanatory since it makes your hand look like a crab when you curl all of your fingers to perform it, according to Qbert the name originated elsewhere. Apparently, he and Mixmaster Mike had just returned from Beirut, Lebanon around the same time that he invented it where they were served crepes one night after a show. He said that when the people over there pronounced crepes, it sounded more like "cccccreb" and since he thought it was funny, he used it to name the "cccccreb" scratch which everyone now pronounces as the crab.