The Collaborative International Dictionary of English v.0.48:

fool around \fool` a*round"\, v. i. [imp. & p. p. Fooled
   around; p. pr. & vb. n. Fooling around.]
   1. To waste time in idle pursuits; to fool[2].

   Syn: fool[2].
        [PJC]

   2. To flirt or philander. --RHUDbr/ [PJC]

   3. To be sexually promiscuous; to engage in sexual
      intercourse with more than one partner; especially, to
      engage in adultery; as, to fool around with another woman.
      [PJC]

WordNet (r) 3.0 (2006):

fool around
    v 1: indulge in horseplay; "Enough horsing around--let's get
         back to work!"; "The bored children were fooling about"
         [syn: horse around, arse around, fool around, fool]
    2: commit adultery; "he plays around a lot" [syn: play around,
       fool around]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "fool around":
   antic, be foolish, be stupid, caper, caracole, carry on, cavort,
   clown, clown around, coquet, curvet, cut a dido, cut capers,
   cut up, dabble, dally, dance, disport, fiddle, fiddle with,
   fiddle-faddle, fidget with, finger with, flirt, flounce, fool,
   fool with, fribble, frisk, frivol, frolic, gambol, get funny,
   go haywire, horse around, idle, invite ridicule, jerk off,
   kid around, loiter, mess around, monkey, monkey around, piddle,
   play, play around, play the buffoon, play the fool, play with,
   potter, putter, rollick, romp, skip, smatter, sport, tinker,
   tomfool, toy, toy with, trifle, trip, twiddle, womanize