Therefore shall evil come upon thee. Connect this with the first clause of Isaiah 47:10, "Thou hast trusted in thine own evil (moral), therefore shall evil (physical) fall upon thee." The same word, ra'ah, is used in both places.
Thou shalt not know from whence it riseth. So the Vulgate, Vitringa, Gesenius, and Dr. Kay. But the bulk of modern commentators (Hitzig, Ewald, Delitzsch, Nagelsbach, Weir, Cheyne) render, "Thou wilt not know how to charm it away."
Both meanings are possible, and are almost equally good; but the parallelism of the clauses is in favour of the latter rendering. Shakhrah should correspond in construction, as in sound, with kapp'rah.
To put it off; literally, to expiate; i.e. to get rid of it by means of expiatory rites. Which thou shalt not know; or, of which thou shalt not be aware. (On the carelessness and want of foresight displayed by the Babylonians, see the comment on Isaiah 47:8.