AAR

Noun

 * 1)  Aircraft accident report.
 * 2)  after-action report; after-action review.

Proper noun

 * 1)   A xis2  A  R chive file (extension  )
 * 2)  Air America Radio
 * 3)  All-American Rejects
 * 4)  American Academy of Religion
 * 5)  Association of American Railroads

Prepositional phrase

 * 1) At any rate.
 * 2)  against all risks.