expression
something extremely rare; something unexpected to exist
Idiomatic expression literally meaning 'jellyfish bones'; used to describe something so rare or unlikely that it is almost unimaginable. Rare term.
彼が遅刻するなんて、まるでくらげの骨だ。
Him being late is like finding jellyfish bones — something you'd never expect.
そんな都合のいい話はくらげの骨のようなものだ。
A story that convenient is like jellyfish bones — exceedingly rare.
The phrase literally means 'jellyfish bones'. Since jellyfish have no bones, the expression is used to refer to something that is extremely rare or unexpected. The exact origin is uncertain, but it is a traditional Japanese idiom.