dour (marked by sternness or harshness; forbidding)
dura mater (the tough fibrous membrane covering the brain and the spinal cord and lining the inner surface of the skull. It is the outermost of the three meninges that surround the brain and spinal cord)
duramen (the older, inactive central wood of a tree or woody plant, usually darker and harder than the younger sapwood)
duress (constraint by threat; coercion)
durum (a hardy wheat used chiefly in making pasta)
endure (to carry on through, despite hardships; undergo)
indurate (to make hard; harden)
obdurate (hardened in wrongdoing or wickedness; stubbornly impenitent)
Definitions are taken from:
The American Heritage® Dictionary of the English Language, Third Edition copyright © 1992 by Houghton Mifflin Company.
Electronic version licensed from InfoSoft International, Inc. All rights reserved.
Webster's Third New International Dictionary, Unabridged. Merriam-Webster, 2002.
http://unabridged.merriam-webster.com (30 Sep. 2006).
English-Word Information: Word Info about English Vocabulary (wordinfo.info).