The Collaborative International Dictionary of English v.0.48:

Rn \Rn\, n. (Chem.)
   the symbol for the chemical element radon.
   [PJC]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "RN":
   LPN, Naval Construction Battalion, Royal Navy, Seabees, USN,
   United States Navy, argosy, armada, charge nurse, coast guard,
   district nurse, division, escadrille, fleet, flotilla,
   graduate nurse, licensed practical nurse, marine, merchant fleet,
   merchant marine, merchant navy, mosquito fleet, naval forces,
   naval militia, naval reserve, navy, nurse, nursing sister,
   practical nurse, private-duty nurse, probationer, probationist,
   probe, public health nurse, registered nurse, school nurse,
   scrub nurse, sister, squadron, student nurse, surgical nurse,
   task force, task group, trained nurse, visiting nurse