I thought this was interesting:
How does a word become a swear word?
As with most things etymological, it's somewhat of a mystery how a term changes from describing a body part or physical act into a disparaged four-letter word. In fact, some modern swear words weren't considered so rude centuries ago. This is due to society's ever-changing ideas about what's taboo. Read on for the answer, but if you're offended by naughty words, steer clear of the links.
As British lexicographer Jonathon Green notes, in medieval England, all the popular swear words were a form of blasphemy. Around 1700, sex and defecation were the genesis for profanity. These days, the most offensive words are racist, sexist, and homophobic terms. While curses involving body parts and functions are common today, their shock value has lessened.
The notorious "f-word" dates back to the 1500s, and it's been impolite ever since. But the familiar "s-word" simply began as a term for diarrhea. More vulgar usages developed over time, with some of the most colorful invented in the 20th century.
How did the evolution (or devolution, some might argue) of such words happen? Very slowly. Invasions and immigration mixed English and other languages, and this helped word meanings change. Much like the definition of obscenity, swearing has changed over time, and a word's usage can vary from place to place.
(from Yahoo)
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