ive seen several definitions here but this is how "teufelshunde"(pl) "teufelshund"(sg) is actually written...the "s" is often forgotten but it is important as it indicates the real meaning of the word which is "dogs of the devil" rather than "devil dogs"
. its an old german slang word for a tough, couragous, risking and in his case devoted person. a "tough bastard" or in some sense also close to "son of a gun"
as you can read in other definitions it comes from ww1 when german soldiers were amazed by the courage and toughness of american soldiers coming into battle at 1917. american soldiers were respected and feared.
Jesus, some Germans, but especially German exchange students are real Teufelshunde. They kick major butt!