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infidelitas

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Etymology and origin of the Latin word infidelitas

Confidence
Established
Formation
Derivation
First attested
81–43 BC, Cicero
Ultimate origin
Latin īnfidēlis
Etymology tree
inherited borrowed derived
Latin infidelis
Latin infidelitas

Forms

īnfidēlitās f (genitive īnfidēlitātis); third declension[1]

Etymology

It is from Latin īnfidēlis (“unfaithful”) + -tās.[2][1][3] Ancient grammarians described it as formed from the dative singular of the adjective.[3]

Earliest recorded meaning

Köbler dates it from Cicero,[4] who speaks of the faithlessness of friendships and asks quantae īnfidēlitātēs in amīcitiīs, “how much faithlessness there is in friendships”.[5]

Semantic evolution

It means faithlessness or inconstancy;[2][5] the TLL finds it in Cicero, Plancus, Caesar and Hirtius, and Cicero uses the plural.[3] Caesar has someone bear the suspicion of disloyalty.[5]

In Christian Latin it is lack of faith, unbelief.[6][7] Medieval Latin used it for disloyalty, an act of treason, and the breaking of a promise or oath.[6] English took infidelity about 1400 for unbelief, from French or Latin, and in the early 15th century for disloyalty to a person.[8]

Timeline

  • 81–43 BC – Cicero uses it of the faithlessness of friends.[5][4]
  • c. 50 BC – Caesar uses it for a suspicion of disloyalty.[5][4]
  • c. 1400 – English borrows it as infidelity, first for unbelief.[8]

Meanings

Noun

  1. Faithlessness, unfaithfulness, inconstancy.[2][5]
  2. (Ecclesiastical Latin) Lack of faith, unbelief.[6][7]
  3. (Medieval Latin) Disloyalty, breach of faith, treason.[6]
  4. (Medieval Latin) Breach of a promise or oath.[6]
  • English infidelity

References

  1. ^ a b Wiktionary, the free dictionary, https://en.wiktionary.org
  2. ^ a b c Glare, P. G. W., ed. (2012), Oxford Latin Dictionary, 2nd ed., Oxford: Oxford University Press
  3. ^ a b c Thesaurus linguae Latinae (1900–), Leipzig: Teubner; Berlin: De Gruyter; open-access ed., Munich: Bayerische Akademie der Wissenschaften
  4. ^ a b c Köbler, Gerhard (2010), Lateinisches Abkunfts- und Wirkungswörterbuch für Altertum und Mittelalter, online ed., koeblergerhard.de
  5. ^ a b c d e f Lewis, Charlton T.; Short, Charles (1879), A Latin Dictionary, Oxford: Clarendon Press
  6. ^ a b c d e Niermeyer, J. F. (1976), Mediae Latinitatis Lexicon Minus, Leiden: Brill
  7. ^ a b Stelten, Leo F. (1995), Dictionary of Ecclesiastical Latin, Peabody, Mass.: Hendrickson
  8. ^ a b Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
Cite this entry

“Infidelitas.” Etymologically, 20 Oct. 2026, etymologically.com/la/infidelitas. Accessed 22 Oct. 2026.

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