un-tem'-perd (taphel): Used of mortar in Eze 13:10-15; 22:28. Taphel probably refers to mortar made with clay instead of slaked lime. In the interior of Palestine and Syria walls are still commonly built of small stones or mud bricks, and then smeared over with clay mortar. The surface is rubbed smooth and is attractive in appearance. This coating prolongs the life of the wall but requires yearly attention if the wall is to stand.
⇒See the definition of untempered in the KJV Dictionary
Ezekiel uses the practice to typify the work of false prophets. They build up stories and make them plausible by an outward semblance to truth, while, in fact, they are flimsy, unreliable prophecies, resembling the walls described above, which can be broken down by a push or a heavy rain storm.
James A. Patch