1630s, "to dull or deaden, make weak" (force, enthusiasm, ardor, etc.), from damp (adj.) + -en (1). Meaning "to moisten, make humid" is recorded from 1827. Related: Dampened; dampening.

"one who or that which dampens," 1857, agent noun from dampen.

late 14c., "to suffocate" (with or as with damp, foul air in a mine), from damp (n.). Figurative meaning "to check or retard the force or action of (the spirits, etc.)" is attested by 1540s. Meaning "to moisten" is recorded from 1670s. Century Dictionary (1897) states that "Dampen is now more common in the literal sense, and is sometimes used in the derived senses." Related: Damped; damping.