The Collaborative International Dictionary of English v.0.48:

Groomsman \Grooms"man\, n.; pl. Groomsmen.
   A male attendant of a bridegroom at his wedding; -- the chief
   attendant is also called the best man; -- the correlative
   of bridesmaid.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Best \Best\ (b[e^]st), a.; superl. of Good. [AS. besta, best,
   contr. from betest, betst, betsta; akin to Goth. batists,
   OHG. pezzisto, G. best, beste, D. best, Icel. beztr, Dan.
   best, Sw. b[aum]st. This word has no connection in origin
   with good. See Better.]
   1. Having good qualities in the highest degree; most good,
      kind, desirable, suitable, etc.; most excellent; as, the
      best man; the best road; the best cloth; the best
      abilities.
      [1913 Webster]

            When he is best, he is a little worse than a man.
                                                  --Shak.
      [1913 Webster]

            Heaven's last, best gift, my ever new delight.
                                                  --Milton.
      [1913 Webster]

   2. Most advanced; most correct or complete; as, the best
      scholar; the best view of a subject.
      [1913 Webster]

   3. Most; largest; as, the best part of a week.
      [1913 Webster]

   Best man, the only or principal groomsman at a wedding
      ceremony.
      [1913 Webster]

WordNet (r) 3.0 (2006):

best man
    n 1: the principal groomsman at a wedding

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "best man":
   acolyte, adjutant, agent, aid, aide, aide-de-camp, aider,
   assistant, attendant, auxiliary, bridemaiden, bridesmaid,
   bridesman, coadjutant, coadjutor, coadjutress, coadjutrix, deputy,
   executive officer, groomsman, help, helper, helpmate, helpmeet,
   lieutenant, matron of honor, paranymph, paraprofessional, second,
   servant, sideman, supporting actor, supporting instrumentalist,
   usher, wedding attendant, wedding party