I once cohabited with an expert in ancient English/Saxon who taught me a good deal about the origins of common swear terms.

Her favourite was a "cock and balls story", which more politely became "cock and bull" or "bull" and then was re- rudified as "bullshit".

She also claimed that "fucare" was perfectly polite Anglo Saxon, meaning "to lie with". "Cunnus" was however, always vulgar Latin, and found its way into other languages. Note that these derivations are quite different from the above.

The history of common words for "shirt" and "toilet" is also very interesting, as words which were polite in one century became very impolite in another and had to be replaced.