[1880–85; ‹ L trīduum period of three days, equiv. to trī- tri- + -duum ‹ *diwom, akin to diēs day (long i perh. after postrīdiē on the following day)]This word is first recorded in the period 1880–85. Other words that entered Englishat around the same time include: barnstorm, impressionism, irredentist, jackpot, pari-mutueltri- is a combining form meaning “three,” used in the formation of compound words. Otherwords that use the affix tri- include: triathlon, triforium, trigeminal, triglyceride, trilemma