…courtesy of a the very informative entry on Beef from The American Heritage Dictionary of the English Language, Fourth Edition, 2000: beef. that was then copied onto Wikipedia:

The word “beef” is from Old French, in contrast to “cow”, which is Germanic. This stems from the Norman Conquest.

After the Norman Conquest, the nobles who ruled England naturally used French words to refer to the meats they were served, while the Germanic words were retained to refer to the live animals.

Thus the animal was called cu (cow) by the Anglo-Saxon peasants—who, it seems, rarely got to eat one—but the meat was called boef (ox) (Modern French boeuf) by the French nobles—who did not often deal with the live animal—when it was served to them for dinner.

This is one example of the common English dichotomy between the words for animals (with largely Germanic origins) and their meat (with Romanic origins) that is also found in such English word-pairs swine/pork, sheep/mutton, and chicken/poultry.

A great example of class shaping language forever.