I don't think dictionary definitions are. However, if I make up a word, that is copyrightable as it is my creation, or IP, or whatever term you wish to use.
Similar to putting a fake street on a map, I would think.
Actually, a fake street is not copyrightable.
http://en.wikipedia.org/wiki/Trap_street#Legal_issues
A United States federal court found that copyright traps are not
themselves protectable by copyright. There, the court
stated: "[t]o treat 'false' facts interspersed among actual facts
and represented as actual facts as fiction would mean that no one
could ever reproduce or copy actual facts without risk of
reproducing a false fact and thereby violating a copyright . . .
. If such were the law, information could never be reproduced or
widely disseminated." (Id. at 733)
One great example is "poonj," meaning to have sexual intercourse. The submitter included some etymological claims, in part that the term originated in the language of the Ojibwe native Americans. I find that pretty suspect, though it did appear on the site for some time before I removed it. The definition and the etymology were printed in the second edition of Cassell's Dictionary of Slang. To their credit, they did include "allegedly".
http://books.google.com/books?id=5GpLcC4a5fAC&pg=PA1120&...
I'm not too concerned about the copyright aspect, though. And actually, I'm still unclear on the extent to which dictionary definitions are copyrightable.