What is the X in X-mas?
Old English ofen "furnace, oven," from Proto-Germanic *ukhnaz (cf. Old Frisian, Dutch oven, Old High German ovan, German Ofen, Old Norse ofn, Old Swedish oghn, Gothic auhns), from PIE *aukw- "cooking pot" (cf. Sanskrit ukhah "pot, cooking pot," Latin aulla "pot," Greek ipnos), originally, perhaps, "something hollowed out." The oven-bird (1825) so called because of the shape of its nest. In slang, of a woman, to have (something) in the oven "to be pregnant" is attested from 1962.
Heb. tannur, (Hos. 7:4). In towns there appear to have been public ovens. There was a street in Jerusalem (Jer. 37:21) called "bakers' street" (the only case in which the name of a street in Jerusalem is preserved). The words "tower of the furnaces" (Neh. 3:11; 12:38) is more properly "tower of the ovens" (Heb. tannurim). These resemble the ovens in use among ourselves. There were other private ovens of different kinds. Some were like large jars made of earthenware or copper, which were heated inside with wood (1 Kings 17:12; Isa. 44:15; Jer. 7:18) or grass (Matt. 6:30), and when the fire had burned out, small pieces of dough were placed inside or spread in thin layers on the outside, and were thus baked. (See FURNACE.) Pits were also formed for the same purposes, and lined with cement. These were used after the same manner. Heated stones, or sand heated by a fire heaped over it, and also flat irons pans, all served as ovens for the preparation of bread. (See Gen. 18:6; 1 Kings 19:6.)