Def: a positive exclamatory remark or compound adjective. Synonymous with da bomb. Refers to an object or event that is clearly the best object or event ever.
I believe that this is a UK slang term. It refers to the fact that a dog can lick his own balls, which seems to be an ideal situation for any single dog. Projected into the human world, the statement heartily endorses things that cannot be improved upon.
"Dude, that party last night was the dog's bollix"