I cannot see the great difficulty which Mr. Halliwell and your
correspondents perceive in the use of this word. Of course they are
aware, that Iscander Dulcarnein (Alexander Bicornis) is Alexander the
Great, the same name being also fabulously ascribed to a far more
ancient and imaginary king; and that the æra of Dulcarnein (or
Macedonian æra) is well known in Eastern chronology. There is therefore
no doubt about the word, only about its application. Why did the name of
this king stand for our Coventry or Jericho, a place to which the people
are flemed or banished?
Because Dulcarnein built the famous iron walls of Jajuge and Majuge,
within which Gog and Magog are confined until the latter days of the
world; when God shall reduce the wall to dust, and set free the captive
nations (_Koran_, cap. xviii.). Sending to Dulcarnein is merely an
ellipsis of the person for his place, _i.e._ for the rampart of
Dulcarnein. Certainly no men can be more effectually flemed than Gog and
Magog were.
But as to the point of being "at one's wits end," no one can be so
little conversant with human affairs as the inmates of the iron wall.
Knowledge depends much on place. So sailors say, "he has been _before_."
I have only an uncommented text of Chaucer. But I cannot understand his
editors allowing this word to "set them at defiance."