1)
What is the meaning of "Yemishun"?
1.
Rashi: It is [feeling,] like "Yemusheni Avi" (Bereishis 27:12).
2)
Why does it say "Lo Yehgu"?
1.
Radak: They do not have even a little Hegyon, i.e. a voice from the throat that is less than speech. This is like "Ehgeh ka'Yonah" (Yeshayah 38:14).
2.
Malbim: They lack even inner speech, a weak chatter between speech and silence.