tricennial（adj.）
"noting 30, occurring once in 30 years," 1650s, from Late Latin tercennium "a space of 30 years." irregularly formed from tricent "thirty each, 30 at a time" (see tri- + cent) + annus "year" (see annual (adj.)). OED cites Blount, Bailey, Webster; perhaps a dictionary word. Related: Tricennarious.
该词起源时间：1650年代