On Yahoo! Answers there was a question asking what word people used instead of "push" before 1800. This slightly odd query came on the evidence of the Ngram Viewer graph, which appears to show virtually no use of the word before the very late 1700s, then a sudden rise into significant use around 1800.
The explanation is simple, though the precise timing is hard to explain. Prior to 1800 or so, printed texts used the "long s" character "ſ" (a.k.a. "medial s" or "descending s"), which Google's OCR algorithm interprets as "f". So if you look instead for "pufh", you find all those missing pre-1800s examples of "push". The transition between the two is striking ...
Wikipedia article describes, it happened at different times in different countries, but just as rapidly as in English.
This phenomenon also explains the strange bipolar Ngram Viewer graph for "fuck".
The post-1960 hits are real. The pre-1800s ones don't represent some robust pre-prudish age, but occurrences of "suck" printed as"ſuck".