Etymograph

English

infidelity

Infidelity comes from Latin īnfidēlitās ‘unfaithfulness, faithlessness, infidelity’, and goes back to Proto-Indo-European *bʰeydʰ- ‘to command, to persuade, to trust’.

  1. Proto-Indo-Europeanc. 4500–2500 BCE*bʰeydʰ-1‘to command, to persuade, to trust’reconstructed
  2. Latinc. 75 BCE–200 CEfidēs2‘faith, trust’
  3. Latinc. 75 BCE–200 CEfidēlis3‘faithful, loyal’
  4. Latinc. 75 BCE–200 CEinfidēlis4‘unfaithful’
  5. Latinc. 75 BCE–200 CEīnfidēlitās5‘unfaithfulness, faithlessness, infidelity’
  6. Middle Frenchc. 1400–1600infidelité5
  7. Middle Englishc. 1150–1500infidelite5
  8. Englishearliest quotation 1674infidelity5

attested reconstructed ? uncertain or disputed deepest recorded ancestor

  1. 1 Wiktionary, Latin fides
  2. 2 Wiktionary, Latin fidelis
  3. 3 Wiktionary, Latin infidelis
  4. 4 Wiktionary, Latin infidelitas
  5. 5 Wiktionary, English infidelity
  6. Earliest quotation in Wiktionary’s entry: 1674, Seth Ward: “The means used to this purpose are partly didactical,…”.

Lineage

  1. Proto-Indo-Europeanc. 4500–2500 BCE*bʰeydʰ-Wiktionary, Latin fides‘to command, to persuade, to trust’reconstructed
  2. Latinc. 75 BCE–200 CEfidēsWiktionary, Latin fidelis‘faith, trust’
  3. Latinc. 75 BCE–200 CEfidēlisWiktionary, Latin infidelis‘faithful, loyal’
  4. Latinc. 75 BCE–200 CEinfidēlisWiktionary, Latin infidelitas‘unfaithful’
  5. Latinc. 75 BCE–200 CEīnfidēlitāsWiktionary, English infidelity‘unfaithfulness, faithlessness, infidelity’
  6. Middle Frenchc. 1400–1600infidelitéWiktionary, English infidelity
  7. Middle Englishc. 1150–1500infideliteWiktionary, English infidelity
  8. Englishearliest quotation 1674infidelityWiktionary, English infidelity. Earliest quotation in Wiktionary’s entry: 1674, Seth Ward: “The means used to this purpose are partly didactical,…”

Linked words

through Proto-Indo-European *bʰeydʰ- ‘to command, to persuade, to trust’

federal, federation, confidence, confident, confederate, abide, defy, defiance, affidavit, bode, fiduciary, foi, confidant, bide, confide, diffident, 27 more

through Latin fidēs ‘faith, trust’

perfidy, 7 more

through Latin fidēlis ‘faithful, loyal’

fealty, 3 more

Today it means

noun Unfaithfulness in a marriage or an intimate (sexual or romantic) relationship: practice or instance of having a sexual or romantic affair with someone other than one's spouse, without the consent of the spouse.

Sources

Sources: Wiktionary, “infidelity”; the older steps come from the Wiktionary entries for Latin fides, Latin fidelis, Latin infidelis, Latin infidelitas. By Wiktionary contributors, CC BY-SA 4.0, via kaikki.org (dump of Oct 3 2026); text adapted and restructured. Language periods are conventional ranges, not dates for this word. License.

Etymograph / Proto-Indo-European *bʰeydʰ- / infidelity

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