Wordorigins.org

Doom is a very old word, dating back to the Old English period. But the Old English dom had a differerent meaning for those in medieval England was quite different than its meaning today. It did not refer to fate or the apocalypse back then, rather it meant a law or judgment at trial.

The word appears as early as c.825 in the Vespasian Psalter with the meaning of a statute, decree, or judgment:

Bioð afirred domas ðine from onsiene his.
(Be afraid, in his presence [are] your dooms)