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.
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.
by railtracksurvivor May 1, 2009
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Crab:
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??
Lol @ the gang war going on on UD. Fighting it out using dictionary definitions, real tough!

Of course, I have full respect for the actual crabs out there under rocks and in the sea and such.
by gratedraindrop February 22, 2007
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An insulting term to crips, mostly used by us bloods
"You fuckin crab, betta not come around this block anymore"
by syked February 18, 2004
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A parasite that lives in the pubic hair. The only cure is to shave off half of the pubic hair, light the other half on fire and stab the little bastards with an ice pick when they come running out.
"I got rid of those crabs but these scars will never heal"
by MoraLes February 20, 2006
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There are two kinds of crabs:

1. The tasty underwater kind

2. The not-tasty underwear kind
1. Bob: Seafood sounds good.
Cindy: Let's go to the store and get crabs to boil and eat tonight.

2. Bob: My junk itches rather badly.
Cindy: You probably have crabs.
by Brittanyk92 April 6, 2009
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When a girl is sending you nudes in this position /\_/\
Bryant: Hey fel, crab for me 🦀
Felicia: What's crabbing
Bryant: /\_/\
by Discord Mob Leader May 16, 2020
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When a man is urinatng in a public urinal on his own and he attempts to urinate an equal amount in each urinal by sidestepping like a crab while using hands to control urine distribution.
"I went into the urinals and there was no one there so I did a bit of crabbing"

"My attempt at crabbing was ruined by someone coming into the toilets and spotting me mid-act, they gave me a pretty dodgy look"
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