Webster’s Revised Unabridged Dictionary · 1913
Devocalize
De·vo′cal·ize (dē·vō′kal·īz), v. t. To make toneless; to deprive of vowel quality. — De·vo′cal·i·za′tion, n.
If we take a high vowel, such as (i) [= nearly i of bit], and devocalize it, we obtain a hiss which is quite distinct enough to stand for a weak (jh). H. Sweet.