1560s, "arrogant, impudent, not modest about one's pretentions," from Latin immodestus "unrestrained, excessive," from assimilated form of in- "not, opposite of" (see in- (1)) + modestus "moderate, keeping due measure, sober, gentle, temperate," from modus "measure, manner" (from PIE root *med- "take appropriate measures"). Meaning "indecent, lewd, not modest in person or utterance" is from 1580s. Related: immodestly.
