1. A large but indefinite quantity, as in He has a good (or great) deal of money; also, to a large extent or degree, as in "I bled ... a great deal" (from Daniel Defoe's Robinson Crusoe, 1719). Also see good many.
2. A very successful transaction or business agreement; a bargain. For example, The new agent got him a great deal, or Only $50,000 for all that land? That's a good deal. [Colloquial; mid-1800s]