Etymology
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surd (adj.)

1550s, "irrational" (of numbers), from Latin surdus "deaf, unheard, silent, dull; willfully deaf, inattentive," possibly related to susurrus "a muttering, whispering" (see susurration). The mathematical sense is from the use of Latin surdus to translate Arabic (jadhr) asamm "deaf (root)," itself apparently a loan-translation of Greek alogos, literally "speechless, without reason" (Euclid book x, Def.). In French, sourd remains the principal word for "deaf." As a noun from 1540s. Related: Surdity.

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Definitions of surd
1
surd (n.)
a consonant produced without sound from the vocal cords;
Synonyms: voiceless consonant
2
surd (adj.)
produced without vibration of the vocal cords;
Synonyms: unvoiced / voiceless / hard
From wordnet.princeton.edu