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Quarantine comes from Medieval Latin quarentena ‘40-day period’, from Latin quadrāgintā ‘forty’, and ultimately from Proto-Indo-European *kʷétwr̥dḱomth₂ ‘forty’.

1. Proto-Indo-Europeanc. 4500–2500 BCE*kʷétwr̥dḱomth₂¹‘forty’reconstructed
2. Latinc. 75 BCE–200 CEquadrāgintā¹‘forty’
3. Late Latinc. 200–600 CE*quaranta¹‘forty’uncertain
4. Medieval Latinc. 600–1500quarentena¹‘40-day period’
5. Middle Englishc. 1150–1500quarentine¹
6. Englishearliest quotation c. 1470quarantine¹

1. ¹ Wiktionary, English quarantine
2. Earliest quotation in Wiktionary’s entry: c. 1470, William Wey, The Itineraries: “By yonde ys a wyldernys of quarentyne, Wher Cryst…”.