expression, noun
crab with little meat; crab of a moonlit night
Literal meaning: a crab caught on a moonlit night, which is said to have little meat. Used as the basis for the idiomatic sense.
月夜の蟹は身が少ないと言われる。
It is said that crabs caught on moonlit nights have little meat.
expression, noun
something with no substance; all show and no substance
Idiomatic use: refers to something that looks impressive but lacks real content or value, like a crab with little meat.
彼の話は月夜の蟹で、中身がなかった。
His story was like a moonlit-night crab—there was nothing to it.
見かけ倒し also means something that looks good but is disappointing in substance, but it is a more general term, while 月夜の蟹 is a vivid idiomatic expression.
The expression originates from the belief that crabs caught on moonlit nights have less meat, possibly because they are more active and use up energy, or because the moonlight makes them easier to catch before they have fed well. The exact origin is uncertain.