1 A unit of brightness of nebulae. A nebula is a cloud-like object visible in the night sky, using a telescope or binoculars; think milky squished star. It - a Crab - is, by definition, equal to the brightness of the Crab Nebula, which was seen as a supernova in 1054 A.D., by Chinese and Arab astronomers; also known as the nebula M1 (Messier1).
A Crab has a sub-division millicrab, which any half-competent student of the metric system will be able to tell you is a thousandth-part of a Crab.
2 A hard-shelled crustacean, with a body generally wider than it is long, two pincers, and a wide variation in size, ranging from pea-crabs, about the size of the eponymous vegetable - to spider crabs, which have a claw span of three or four metres/yards in large (= old) specimens.
3 Pubic lice. This indicates that you may not have been too careful aboutyour fuck-buddies.
2 Crabs can be caught in many coastal waters - but be careful picking them up, as they can nip.
3 'That damned soccer-jock gave me crabs - he must have shagged Brutus last week!' said Pandora with resignation, whilst counting her plentiful takings.
A strange creature with weak body armour and has a funny walk. It looks dangerous because it has weapons on the end of its foremost arms but these can easily be overcome by common sense. It can't read, write or talk properly because it is a crustacean....
Does that sound anything like the vast majority of authors for the definition of "Crab", and the ones to whom they apply their definitions??
Of course, I have full respect for the actual crabs out there under rocks and in the sea and such.