The Collaborative International Dictionary of English v.0.48:

Pa \Pa\ (p[aum]), n.
   A shortened form of Papa.
   [1913 Webster]

WordNet (r) 3.0 (2006):

p.a.
    adv 1: by the year; every year (usually with reference to a sum
           of money paid or received); "he earned $100,000 per
           annum"; "we issue six volumes per annum" [syn: per
           annum, p.a., per year, each year, annually]
    n 1: an electronic amplification system used as a communication
         system in public areas [syn: public address system, P.A.
         system, PA system, P.A., PA]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "pa":
   abba, dad, daddy, father, foster father, genitor, governor,
   old man, pap, papa, pappy, pater, paterfamilias, patriarch, pop,
   pops, sire, stepfather, the old man



Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "PA":
   Gramophone, PA system, Victrola, audio sound system, audiophile,
   binaural system, bitch box, bullhorn, cartridge, ceramic pickup,
   changer, crystal pickup, derived four-channel system,
   discrete four-channel system, four-channel stereo system, hi-fi,
   hi-fi fan, high-fidelity, intercom, intercommunication system,
   jukebox, magnetic pickup, monaural system, mono, needle,
   nickelodeon, phonograph, photoelectric pickup, pickup,
   public-address system, quadraphonic sound system,
   radio-phonograph combination, record changer, record player,
   sound reproduction system, sound truck, squawk box, stereo, stylus,
   system, tape deck, tape recorder, tone arm,
   transcription turntable, turntable



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

PA
       Precision Architecture (HP)