The "un" in unless isn't the negating prefix "un-"; it's the word "on" with a change of vowel. The etymology is "on less[er condition than]," e.g., "Abort the l

The "un" in unless isn't the negating prefix "un-"; it's the word "on" with a change of vowel. The etymology is "on less[er condition than]," e.g., "Abort the launch on less than the rocket is deemed safe to fly."