According to https://www.etymonline.com/word/dampen https://www.etymonline.com/word/dampen it's meant "to dull or deaden, make weak" from 1630s and "to moisten,

According to https://www.etymonline.com/word/dampen https://www.etymonline.com/word/dampen it's meant "to dull or deaden, make weak" from 1630s and "to moisten, make humid" from 1827.