*devocalize*

v. t. To make toneless; to deprive of vowel quality. -- De*vo`cal*i*za"tion, n. [1913 Webster]

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. [1913 Webster]