Surd (?), a. [L. surdus deaf (whence the meaning, deaf to reason, irrational), perhaps akin to E. swart. Cf. Sordine.] 1. Net having the sense of hearing; deaf. “A surd... generation.” Sir T. Browne.
2. Unheard. Kenrick.
3. (Math.) Involving surds; not capable of being expressed in rational numbers; radical; irrational; as, a surd expression or quantity; a surd number.
4. (Phonetics) Uttered, as an element of speech, without tone, or proper vocal sound; voiceless; unintonated; nonvocal; atonic; whispered; aspirated; sharp; hard, as f, p, s, etc.; — opposed to sonant. See Guide to Pronunciation, §§169, 179, 180.