The Collaborative International Dictionary of English v.0.48:

Halidom \Hal"i*dom\ (h[a^]l"[i^]*d[u^]m), n. [AS. h[=a]ligd[=o]m
   holiness, sacrament, sanctuary, relics; h[=a]lig holy +
   -d[=o]m, E. -dom. See Holy.]
   1. Holiness; sanctity; sacred oath; sacred things; sanctuary;
      -- used chiefly in oaths. [Archaic]
      [1913 Webster]

            So God me help and halidom.           --Piers
                                                  Plowman.
      [1913 Webster]

            By my halidom, I was fast asleep.     --Shak.
      [1913 Webster]

   2. Holy doom; the Last Day. [R.] --Shipley.
      [1913 Webster]