late 14c., originally of weather and other physical conditions, from Latin moderatus "within bounds, observing moderation;" figuratively "modest, restrained," past participle of moderari "to regulate, mitigate, restrain, temper, set a measure, keep (something) within measure," from PIE *med-es-, from root *med- "take appropriate measures." The notion is "keeping within due measure." In English, of persons from early 15c.; of opinions from 1640s; of prices from 1904. Related: Moderateness.