1551, "irrational" (of numbers), from L. surdus
"unheard, silent, dull," possibly related to susurrus
"a muttering, whispering" (see susurration
). The mathematical sense is from the use of L. surdus
to translate Ar. (jadhr) asamm
"deaf (root)," itself a loan-translation of Gk. alogos,
lit. "speechless, without reason" (Euclid bk. x, Def.). In Fr., sourd
remains the principal word for "deaf."