surd

Etymology

Borrowed from Latin surdus (“deaf”); in mathematical sense, "deaf to reason", i.e. irrational.

noun

1. (arithmetic) An irrational number, especially one expressed using the √ symbol.
2. (linguistics) A voiceless consonant.

adj

1. (obsolete) Lacking the sense of hearing; deaf.
   …how all Words fall to the Ground, spent upon such a surd and Earless Generation of Men, stupid unto all Instruction… 1670s, published 1716, Thomas Browne, Christian Morals, part 3, section 6
2. (obsolete) unheard
   To this errour, of blending the ſurd and vocal modes of articulation together, may be added the too frequent uſe of compound articulations both vocal and ſurd. 1773, William Kenrick, A New Dictionary of the English Language, section 3, page 5
3. (mathematics) Involving surds, or irrational numbers; not capable of being expressed in rational numbers.
   a surd expression or quantity; a surd number
4. (phonetics) unvoiced; voiceless