Etymograph

English

thorough

Thorough comes from Old English þurh ‘through’, and perhaps goes back to Proto-Indo-European *terh₂-.

  1. Proto-Indo-Europeanc. 4500–2500 BCE*terh₂-1‘to pass through’uncertain
  2. Proto-Germanicc. 500 BCE–200 CE*þurhw1‘through’reconstructed
  3. Old Englishc. 450–1150þurh2‘through’
  4. Old Englishc. 450–1150þuruh2byform of þurh
  5. Middle Englishc. 1150–1500thoruȝ2
  6. Englishearliest quotation 1590thorough2

attested reconstructed ? uncertain or disputed deepest recorded ancestor

  1. 1 Wiktionary, Old English þurh
  2. 2 Wiktionary, English thorough
  3. Earliest quotation in Wiktionary’s entry: 1590, Edmund Spenser, Book II, Canto XII: “Ye might haue ſeene the frothy billowes fry Vnder…”.

Lineage

  1. Proto-Indo-Europeanc. 4500–2500 BCE*terh₂-Wiktionary, Old English þurh‘to pass through’uncertain
  2. Proto-Germanicc. 500 BCE–200 CE*þurhwWiktionary, Old English þurh‘through’reconstructed
  3. Old Englishc. 450–1150þurhWiktionary, English thorough‘through’
  4. Old Englishc. 450–1150þuruhWiktionary, English thoroughbyform of þurh
  5. Middle Englishc. 1150–1500thoruȝWiktionary, English thorough
  6. Englishearliest quotation 1590thoroughWiktionary, English thorough. Earliest quotation in Wiktionary’s entry: 1590, Edmund Spenser, Book II, Canto XII: “Ye might haue ſeene the frothy billowes fry Vnder…”

The story

The adjective derives from the preposition and adverb. The word developed a syllabic form in cases where the word was fully stressed: when it was used as an adverb, adjective, or noun, and less commonly when used as a preposition.

Full etymology on Wiktionary

Linked words

through Proto-Indo-European *terh₂- ‘to pass through’

enter, trans, avatar, tram, 23 more

through Proto-Germanic *þurhw ‘through’

throughout

through Old English þurh ‘through’

through, 1 more

Today it means

adj Painstaking and careful not to miss or omit any detail.

Sources

Sources: Wiktionary, “thorough”; the older steps come from the Wiktionary entry for Old English þurh. 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 *terh₂- / thorough

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