은어가 · 은어를
slang, jargon, secret language of a group
Example
이 단어는 군대에서 쓰는 은어예요.
This word is military slang.
Word parts
은 hidden+어 word
Literally 'hidden words', understood only inside a group.
Good to know
Homonym warning: 은어 is also the fish 'sweetfish'; context makes it clear.