Translation of Chinese word is not an English word? This is really strange. Dictionary.com says "obdurability" isn't a word in English, but it is the ONLY translation for "强韧" in Pleco (my Chinese-English dictionary). Google.com has tons of results, but almost all of them seem to be Chinese companies describing their durable materials as having "obdurability"! Was this a typo by someone compiling a dictionary for Chinese readers that just got magnified over the years or what?
The ABC dictionary definition in Pleco is "pliable but tough". Other dictionaries in my Pleco edition have the same definition.  The only one that has yours is ADSO which is a community dictionary provided by the people of Popup Chinese. 

My hypothesis about the wide-spread use of this word:
In online Chinese dictionaries I often see non-existing English words being used in the translation. This seem to happen when the Chinese word doesn't exist in English and hence can't be translated by one word. There is a lot of copied material on Chinese websites. You for example also see this on the Chinese wiki alternatives. The texts on different sites on a certain topic are most of the time almost complete rip-offs. It seems that they just want to be as complete as possible with minimal effort and in this way errors are spread quite quickly.