surd

Etymology

It is borrowed from Latin surdus (“deaf; making no clear sound, dull, indistinct”).^{[2][3][4][5][6]} The mathematical sense comes from the use of the Latin word to render Ancient Greek ἄλογος (álogos, “speechless; irrational”), Euclid's term for incommensurable quantities,^[2][5][7][8] apparently by way of Arabic جِذْر أَصَمّ (jaḏr aṣamm, “deaf root”), which the Latin translates.^[2][3][8][1] Barnhart, Klein and Merriam-Webster Unabridged relate the Latin adjective to susurrus (“a whisper, murmur”),^[3][7][6] which Klein derives from the imitative root Proto-Indo-European *swer- (“to sound”);^[7] Etymonline finds the link awkward in sense, and cites de Vaan's speculation that surdus took its meaning "not hearing" from absurdus.^[4] The Latin word survives as French sourd (“deaf”).^[4][2]

Earliest recorded meaning

Of a number or quantity, especially a root: irrational, not expressible in ordinary numbers: 1551, in Robert Recorde's Pathway to Knowledge.^[2][3] Etymonline gives the 1550s,^[4] and Dictionary.com 1545–55.^[9]

Semantic evolution

The mathematical adjective was soon used as a noun for a surd number or root (1557, in Recorde's Whetstone of Witte).^[2][3][6] The 17th century tried the Latin senses too: "deaf" (from before 1682, in Sir Thomas Browne, now obsolete) and, figuratively, "insensate" (1601–76) and "senseless, irrational" (from 1610), the last later felt as a figure from the mathematical sense.^[2] Johnson (1755) defined it only as deaf, unheard, and "not expressed by any term".^[10] In phonetics the word has meant "voiceless" since 1767, and the noun a voiceless consonant since 1789, in Erasmus Darwin, opposed to sonant;^[2][3][5] the OED compares the French use of sourd.^[2] Translating the Arabic grammarians' term, it was also applied from 1776 to Arabic verbs whose second and third root letters are the same.^[2] The rare verb surd, to muffle a sound as with a mute, is recorded from 1625.^[2][11]

Timeline

• 1551 – of a number, irrational.^[2][3]
• 1557 – noun, an irrational number or root.^[2][3][6]
• 1601 – insensate, without perception (obsolete by 1676).^[2]
• 1610 – irrational, senseless.^[2][6]
• 1625 – verb, to deaden the sound of.^[2]
• before 1682 – deaf (obsolete, rare).^[2]
• 1767 – in phonetics, voiceless.^[2][3]
• 1776 – in Arabic grammar, of a verb with the same second and third root letters.^[2]
• 1789 – noun, a voiceless consonant.^[2][3]

Meanings

Noun

1. (mathematics) An irrational root of a rational number, or an expression containing one, such as √3.^[12][6][2]
2. (phonetics) A voiceless consonant, such as t.^[12][6][2]
3. Something irrational or unknown at the heart of a matter.^[6]

Adjective

1. (mathematics) Of a number or root: irrational, not expressible as a ratio of whole numbers.^[12][2][6]
2. (phonetics) Of a speech sound: uttered without vibration of the vocal cords.^[12][6][2]
3. Lacking sense or reason: the surd cruelty of chance.^[6][2]
4. (obsolete) Deaf.^[12][2][10]

Verb

1. (rare) To deaden the sound of, as with a mute.^[11][2]

Related words

• absurd
• sordino
• sourdine
• susurration
• surdity

Usage notes

The OED also records surd, to defile, once before 1400; it has a separate source, Old English *seordan, and is otherwise found as sard.^[2]

References

1. ^ a b c Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z 27 28 Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b c d e f g h i j Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
4. ^ a b c d Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
5. ^ a b c Hoad, T. F., ed. (1986), The Concise Oxford Dictionary of English Etymology, Oxford: Clarendon Press
6. ^ a b c d e f g h i j k Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
7. ^ a b c Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. 2 vols. Amsterdam: Elsevier, 1966–1967.
8. ^ a b Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press
9. ^ Dictionary.com Unabridged (2016), based on the Random House Unabridged Dictionary, New York: Random House
10. ^ a b Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
11. ^ a b Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
12. ^ a b c d e Collins English Dictionary (2023), Glasgow: HarperCollins