American Dictionary of the English Language

SURD, adjective [Latin surdus, deaf.] Deaf; not having the sense of hearing. [Not used.]

1. Upheard. [Not used.]

2. Designating a quantity whose root cannot be exactly expressed in numbers.

SURD, noun In algebra, a quantity whose root cannot be exactly expressed in numbers. Thus 2 is a surd number, because there is no number which multiplied into itself, will exactly produce 2.