The word sound showing approvement and acknowlegdement and pound to show whatever you are refering to. Therefore showing approvement towards something. Originally from the days when the pound coin was introduced was said around London as there was a good exchange rate.
"The party was sound as a pound". Showing good humoured feelings towards it.
A person who may not be astute but has a seemingly purposeful presence. The person may be useful but doesn't have an external awareness. A mop serves a purpose but is not a intelligectually significant.