(CAUTION: EXPLICIT SEXUAL CONTENT IN LINKS BELOW)
Searching the Wikimedia Commons for the word "purpose":
Returns this file as the first result:
Investigation into the matter (http://commons.wikimedia.org/wiki/Commons:Village_pump#Bizarre_search_result) has revealed that the word "purpose" is not in the file's description, EXIF, or any backlinks.
A suggestion has been made that it may somehow be related to the word "pose", which several other files in the same series include in their name, such as this (also explicit):
That makes me wonder if this is somehow related to bug 2511, from way back in 2005, in that it could relate to stemming, but that's pure guessing.