late Old English þræl "bondman, serf, slave," from a Scandinavian source akin to Old Norse þræll "slave, servant," figuratively "wretch, scoundrel," probably from Proto-Germanic *thrakhilaz, literally "runner," from root *threh- "to run" (source also of Old High German dregil "servant," properly "runner;" Old English þrægan, Gothic þragjan "to run"). Meaning "condition of servitude" is from early 14c.
-dom
abstract suffix of state, from Old English dom "statute, judgment" (see doom (n.)). Originally an independent word, but already active as a suffix in Old English (as in freodom, wisdom). Cognate with German -tum (Old High German tuom). "Jurisdiction," hence "province, state, condition, quality."