Just recently I heard the word "surd" for the first time. My initial thought was that it was a slang-like shortening of the word "absurd". How wrong I was.
According to Answers.com, the word "surd" has the following two definitions:
1. Mathematics. An irrational number, such as √2.
2. Linguistics. A voiceless sound in speech.
I am amazed that in all of my linguistics studies and research I have never encountered the word. On the other hand, I guess I shouldn't be that amazed because a google search returns results that primarily pertain to mathematics.
Digging a bit deeper, the Online Etymology Dictionary includes this history:
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."