Three accepted meanings:
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.
Example:
1 The new
nebula is about five hundred and twenty millicrabs - say half the brightness of the Crab
Nebula - said the little blonde astronomer; plainly, she didn't seek to
sensationalise this new discovery.
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.