Definition from Wiktionary, the free dictionary
Etymology 1 
- simple past tense and past participle of classify
- Make sure all the field observations are classified by species.
- National security information formally assigned by governments to one of several levels of sensitivity, usually (in English) top secret, secret, confidential, and, in some countries, restricted; thereby making disclosure to unauthorized persons illegal.
- We do not discuss specific interrogation techniques because they are classified information.
- Any information a person or organization does not wish to disclose.
- I won't say who I'm going to the prom with; that's classified.
Etymology 2 
From classified advertisement, by shortening.
classified (plural classifieds)
- (informal) A classified advertisement in a newspaper or magazine.
- He ran a classified for the tools he wanted to sell.
classified advertisement — see classified advertisement