Etymology
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classified (adj.)

1828, "arranged in classes," past-participle adjective from classify. Meaning "secret" (in reference to government information) is from 1941, American English. Classifieds (n.) "newspaper advertisements arranged by classes," 1913, is short for classified advertisements

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Definitions of classified
1
classified (adj.)
arranged into classes;
classified (adj.)
official classification of information or documents; withheld from general circulation;
2
classified (n.)
a short ad in a newspaper or magazine (usually in small print) and appearing along with other ads of the same type;
Synonyms: classified ad / classified advertisement
From wordnet.princeton.edu