What is the X in X-mas?
early 14c., "a noxious vapor," perhaps in Old English but there is no record of it. If not, probably from Middle Low German damp; ultimately in either case from Proto-Germanic *dampaz (cf. Old High German damph, German Dampf "vapor;" Old Norse dampi "dust"). Sense of "moisture, humidity" is first certainly attested 1706.
late 14c., "to suffocate," from damp (n.). Figurative meaning "to deaden (the spirits, etc.)" attested by 1540s. Meaning "to moisten" is recorded from 1670s. Related: Damped; damping.
1580s, "dazed," from damp (n.). Meaning "slightly wet" is from 1706. Related: Dampness.
any of various harmful vapours produced during mining operations. The gases are frequently called damps (German Dampf, "vapour"). Firedamp is a gas that occurs naturally in coal seams. The gas is nearly always methane (CH4) and is highly inflammable and explosive when present in the air in a proportion of 5 to 14 percent. White damp, or carbon monoxide (CO), is a particularly toxic gas; as little as 0.1 percent can cause death within a few minutes. It is a product of the incomplete combustion of carbon and is formed in coal mines chiefly by the oxidation of coal, particularly in those mines where spontaneous combustion occurs. Black damp is an atmosphere in which a flame lamp will not burn, usually because of an excess of carbon dioxide (CO2) and nitrogen in the air. Stinkdamp is the name given by miners to hydrogen sulfide (H2S) because of its characteristic smell of rotten eggs. Afterdamp is the mixture of gases found in a mine after an explosion or fire