The Collaborative International Dictionary of English v.0.48:

Par \Par\, n. [L. par, adj., equal. See Peer an equal.]
   [1913 Webster]
   1. Equal value; equality of nominal and actual value; the
      value expressed on the face or in the words of a
      certificate of value, as a bond or other commercial paper.
      [1913 Webster]

   2. Equality of condition or circumstances.
      [1913 Webster]

   3. An amount which is taken as an average or mean. [Eng.]
      [Webster 1913 Suppl.]

   4. (Golf) The number of strokes required for a hole or a
      round played without mistake, two strokes being allowed on
      each hole for putting. Par represents perfect play,
      whereas bogey makes allowance on some holes for human
      frailty. Thus if par for a course is 75, bogey is usually
      put down, arbitrarily, as 81 or 82. If par for one hole is
      5, a bogey is 6, and a score of 7 strokes would be a
      double bogey.
      [Webster 1913 Suppl. +PJC]

   At par, at the original price; neither at a discount nor at
      a premium; -- used especially of financial instruments,
      such as bonds.

   Above par, at a premium.

   Below par,
      (a) at a discount.
      (a) less than the expected or usual quality; -- of the
          quality of objects and of the performance of people;
          as, he performed below par in the game.

   On a par, on a level; in the same condition, circumstances,
      position, rank, etc.; as, their pretensions are on a par;
      his ability is on a par with his ambition.

   Par of exchange. See under Exchange.

   Par value, nominal value; face value; -- used especially of
      financial instruments, such as bonds.
      [1913 Webster +PJC]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "below par":
   ailing, at a discount, at a reduction, bad, below, below deck,
   below standard, below the mark, belowstairs, critically ill, down,
   down below, downstairs, faint, faintish, feeling awful,
   feeling faint, feeling something terrible, ill, in danger,
   indisposed, infra, laid low, mortally ill, not quite right, off,
   off-color, out of sorts, rocky, seedy, sick, sick unto death,
   sickish, substandard, taken ill, under, under the weather,
   underfoot, unwell