The Collaborative International Dictionary of English v.0.48:

Bygone \By"gone`\ (b[imac]"g[o^]n`; 115), a.
   Past; gone by. "Bygone fooleries." --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bygone \By"gone`\, n.
   Something gone by or past; a past event. "Let old bygones be"
   --Tennyson.
   [1913 Webster]

   Let bygones be bygones, let the past be forgotten.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bygone
    adj 1: well in the past; former; "bygone days"; "dreams of
           foregone times"; "sweet memories of gone summers";
           "relics of a departed era" [syn: bygone, bypast,
           departed, foregone, gone]
    n 1: past events to be put aside; "let bygones be bygones" [syn:
         bygone, water under the bridge]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "bygone":
   ago, antiquated, antique, archaic, belated, blown over, by, bypast,
   dated, dead, dead and buried, deceased, defunct, departed, elapsed,
   erstwhile, expired, extinct, finished, forgotten, former, gone,
   gone glimmering, gone-by, has-been, irrecoverable, lapsed, late,
   lost, no more, obsolete, of old, of yore, old, old-time, old-timey,
   olden, oldfangled, once, onetime, out-of-date, over, passe, passed,
   passed away, past, quondam, run out, sometime, vanished, whilom,
   wound up