William the Conqueror defeated King Harold in the Battle of Hastings in 1066, changing the way Englishmen would speak — and cuss — for all of time.
Before the war William was the Duke of Normandy, the area of western France that the Allies would invade almost 900 years later to defeat the Nazis. William had ambitions beyond the continent though, and sought out an audience with King Edward the Confessor of England. Edward had no children and no obvious heir.
William claimed that Edward promised him the throne during this conversation in 1060, but on his deathbed Edward named an English noble as his successor. Harold Godwine ascended to the throne in 1066 but William immediately called bull-scheisse and contested Harold's claim.
For obvious reasons, Harold wasn't eager to give up his throne. So William crossed the English channel with 7,000 soldiers. Harold had just finished fighting off a Norwegian invasion and was forced to face this new threat with a diminished number of troops.
About two weeks later, Harold and William met with their armies near Hastings. The battle raged all day Oct. 13, 1066, and Harold was killed at the end of the fighting.
The Normans marched to London and William was crowned king. Once he ascended, William declared French, his native language, the official language of the court. This left the Germanic language spoken by the Anglo-Saxons, Old English, as a "lower" language.
According to the Oxford Dictionary blog, this created a two-tiered language that evolved into modern English. Words for things connected to the ruling class, like large homes and prepared meat, drew from French. So noblemen lived in mansions and ate buef (beef) and porc (pork), while an Anglo-Saxon lived in hus (houses) and raised cus (cows) and picg (pigs).
When it came to the lowest and most vulgar of words, like those for poop, butts, and sex, the Old English words with German roots, scheisse, arsch, and ficken, became terms of profanity. It shouldn't be too hard to guess which words those evolved into.