dampen (v.)

1580s, "dazed," from damp (n.). Meaning "slightly wet" is from 1706. Related: Damply; dampness.

word-forming element making verbs (such as darken, weaken) from adjectives or nouns, from Old English -nian, from Proto-Germanic *-inojan (also source of Old Norse -na), from PIE adjectival suffix *-no-. Most active in Middle English and early modern English, hence most verbs in -en are comparatively recent.

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