Surd

a. [Lat. surdus, Pg. surdo, Sp. & It. sordo, Pr. sord, sort, O. Fr. sort, N. Fr. sourd.]

1. Not having the sense of hearing; deaf. [Obs.]
2. Unheard. [Obs.] “Surd modes of articulation.” Kenrick.
3. (Math.) Involving surds; not capable of being expressed in rational numbers; surdal; radical; as, a surd expression or quantity; a surd number.
4. (Pron.) Uttered with simple breath; not sonant; unintonated; toneless; atonic; aspirate; devoid of proper vocality;—said of articulate sounds, or alphabetic letters, as s, f, p, compared with z, v, b, which are sonant, or intonated, or vocal.