An often used phrase in the North West of England, it refers to anything that is old, redundant, slow, or obselete. A Lusitano Nag can be an item that is no longer up to date, an idea that is no longer valid or a person who has done something irrevocably stupid.
The phrase originates from the horse breed, Lusitano. The Lusitano is considered one of the best breeds in the world, but once old, like a car or aging prostitute, its value decreases rapidly before it is thrown to the knackers yard.
'Why did you eat my dinner, you Lusitano Nag?' - Said to a person who has eaten another's dinner.
'Come on, start you Lusitano Nag!' - Said by a person attempting to start their banger on a cold wintry day.