Trimester

google

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early 19th century: from French trimestre, from Latin trimestris, from tri- ‘three’ + mensis ‘month’.

wiktionary

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From French trimestre, from Latin trimestris(“of three months”), from trēs(“three”) + mensis(“month”).

etymonline

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trimester (n.)

1821, "period of three months," from French trimestre (early 17c.), from Latin trimestris "of three months," from tri- "three" (see tri-) + mensis "month" (see moon (n.)). Specific obstetrics sense is attested from 1900. Related: Trimestrial.