(astronomy) A constellation of the zodiac supposedly shaped like a crab.
(astrology) The zodiac sign for the crab, ruled by the Moon and covering June 22 - July 22 (tropical astrology) or July 16 - August 15 (sidereal astrology).
Someone with a Cancer star sign.
(medicine, oncology, pathology) A disease in which the cells of a tissue undergo uncontrolled (and often rapid) proliferation.
(figuratively) Something damaging that spreads throughout something else.
(slang) Extremely unpleasant and annoying.