To keep on working to the end. The analogy of a draft horse working until it drops dates from Shakespeare’s time (or earlier). “At least we’ll die with harness on our back,” says Macbeth before his fateful battle with Macduff (Macbeth, 5.5). Such a death, incidentally, is considered desirable and admirable. “It is a man dying with his harness on that angels love to escort upward,” said the American preacher Henry Ward Beecher (Proverbs from Plymouth Pulpit, 1887). Precisely the same is meant by to die with one’s boots on, although more likely this expression comes from the battlefield (soldiers dying on active duty).