UPDATE: Dami, in a comment to this post, says “if a word appears in a newer edition of an older work (e.g. in the introduction section of cheap reprints of public domain books) Google will count it as an appearance at the time the original work was published.” I checked and this is true, thanks Dami!
I was playing with Google Books Ngram Viewer, which allows you to check how frequently certain phrases occurred in books published since 1950 up to 2008.
Curiously the following graph reports that some books (only 0.0000011% but greater than zero anyway!) were containing the work “wikipedia” (and “wiki”) already in 1950 and in 1975. Maybe there is a small bug even in mighty google services?