Slug means any kind of bullet or round in most urban areas. The proper meaning when talking about amunition is a solid shot gun round that is fired from a rifled bore shot gun, making slugs the most accurate type of shot gun round. Rifled slugs are also available for shooting slugs through smooth bore shot guns. In the Baltimore metropolitan area, a slug means a dollar, typically a dollar bill. It's usually used when asking or stating to have or borrow a dollar rather than stating the cost of something that is worth a dollar.
Let me hold a slug so I can get a rillo from the store. -Baltimore region.
Shawn caught a slug last night. The doctor said he might not make it. -Most urban regions