"exceeding the usual or proper limit, degree, measure, or proportion; going beyond what is sanctioned by correct principles; immoderate; extravagant; unreasonable;" late 14c., from Old French excessif "excessive, oppressive," from Latin excess-, past-participle stem of excedere "to depart, go beyond" (see exceed). Related: Excessively; excessiveness.
early 15c., "to abate excessiveness, reduce the intensity of;" 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 root *med- "take appropriate measures." Intransitive sense of "become less violent, severe, rigorous, etc." is from 1670s. Meaning "to preside over a debate" is first attested 1570s. Related: Moderated; moderating.