Classified

google

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late 18th century: back-formation from classification, from French, from classe ‘class’, from Latin classis ‘division’.

wiktionary

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classify +‎  -ed

From classified advertisement, by shortening.

etymonline

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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