I. ˈsərd adjective
Etymology: Latin surdus deaf, silent, stupid
Date: 1610
1. : lacking sense : irrational
surd conceits of scripture's sense — Thomas Jackson
2. : voiceless — used of speech sounds
II. noun
Date: 1557
1.
a. : an irrational root (as √3)
b. : irrational number
2. : a surd speech sound