What is the X in X-mas?
past participle suffix of weak verbs, from Old English -ed, -ad. --od (leveled to -ed in Middle English), from Proto-Germanic *-do- (cf. Old High German -ta, German -t, Old Norse -þa, Gothic -da, -þs), from PIE *-to- (cf. Sanskrit -tah, Greek -tos, Latin -tus).
Originally fully pronounced, as still in beloved (which, with blessed, accursed, and a few others retains the full pronunciation through liturgical readings). In 16c.-18c. often written -t when so pronounced (usually after a consonant or short vowel), and still so where a long vowel in the stem is short in the pp. (e.g. crept, slept, etc.). In some older words both forms exist, with different shades of meaning, e.g. gilded/gilt, burned/burnt.
witness, a word not found in the original Hebrew, nor in the LXX. and Vulgate, but added by the translators in the Authorized Version, also in the Revised Version, of Josh. 22:34. The words are literally rendered: "And the children of Reuben and the children of Gad named the altar. It is a witness between us that Jehovah is God." This great altar stood probably on the east side of the Jordan, in the land of Gilead, "over against the land of Canaan." After the division of the Promised Land, the tribes of Reuben and Gad and the half-tribe of Manasseh, on returning to their own settlements on the east of Jordan (Josh. 22:1-6), erected a great altar, which they affirmed, in answer to the challenge of the other tribes, was not for sacrifice, but only as a witness ('Ed) or testimony to future generations that they still retained the same interest in the nation as the other tribes.