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."