The Oxford Companion to American Food and Drink

Bully beef [corned beef]

"Bully beef" is the name used primarily by British and British colonial soldiers for canned corned beef. Most sources indicate that the name came into use during World War I when troops noticed the words bouilli bœuf (French for "boiled beef") on the cans. However, an Australian source says the term derived from the canned salt beef supplied from Booyoolee Station to man working in the outback in the 1870s. The workers called this ration "Booyoolee beef", which was shortened to bully beef.
     Although references to bully beef turn up chiefly in British, South African, Australian, Jamaican, New Zealand and Canadian sources, American soldiers also used the term. In 1932 a group of World War I veterans formed the Last Buddies' Bully Beef Club, whose symbol was a bottle of cognac and a can of bully beef. During World War II one Michigan soldier wrote that the only rations that survived a supply-plane drop were the cans of bully beef.