The nickname Impessa was given to Baden-Powell during the Matabele campaign in South Africa during which he famously hunted Dinuzulu kaCetshwayo, king of the Zulu nation. His pursuit of the chieftain was relentless and Baden-Powell's native guides began to call him Impeesa which was translated to him as "the beast that does not sleep, but walks about at night."
The nickname stuck with him. During the siege of Mafeking, it was further translated as "The wolf that never sleeps," a tribute to Baden-Powell's reputation as a watchful military leader and scout. At Mafeking, a cannon built by the defenders during the siege was even called "The Wolf" in his honor.
The true meaning of the word "Impeesa" though, is actually hyena. But being called a hyena is not a particularly complimentary term, so the cunning qualities of the hyena were imposed on a like—and decidedly more noble beast—the wolf. Which actually had no meaning in Africa at all as there are no wolves on the African Veldt!
To quote the legendary commentator, Paul Harvey, "So now you know the rest of the story."
Good night, gentlemen.