The Germans insisted on using the term "unrestricted submarine warfare" when what they really meant was that they would be disobeying the rules of naval combat (yeah, even wars have rules), and blowing up unarmed ships without warning, including merchant and passenger ships. They were angry that the U.S. and other "neutral" countries were selling arms to the Allied nations and not to the Central Powers.

So they wanted to use submarines to blow up all the ships that might be helping their enemies. That's the submarine warfare part. And just to be safe, they decided to blow up all the ships they came across, leading them to call it unrestricted.

The lesson here is: if you want to want to warn people about them big explosions you're about to make, use really long words and you'll sound diplomatic. Some contemporary examples? How about "collateral damage"? That means dead civilians. Or "enhanced interrogation"? Hello, torture.