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. [ Obs. ] “A surd . . . generation.” Sir T. Browne. [ 1913 Webster ]
2. Unheard. [ Obs. ] Kenrick. [ 1913 Webster ]
3. (Math.) Involving surds; not capable of being expressed in rational numbers; radical; irrational; as, a surd expression or quantity; a surd number. [ 1913 Webster ]
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. [ 1913 Webster ]