A slider actually is a coin that is most likely extremely high level AU, and could on any given day be graded MS. Classic examples of this are AU 58 coins---I have seen them grade anywhere from Au 55 to MS 63. It depends on whether the TPG finds high spot wear, or sees it as cabinet/storage friction. Usually, they can be extremely nice coins that end up in the AU 58 category.
So many are sadly hung up on the "higher numerical grade" is better approach, which is sad. Like you, I have handled some amazing AU58s that are much, much more attractive than a baggy MS60-MS62.
I have posted many times some of my key date sliders--I have several AU 58s that I am certain that could crack out and grade as MS, but I am happy with them right where they are. Two of my key date Peace Dollars are really attractive AU 58s.
The origin for the term slider is a coin that is inferior to uncirculated coins AND is being mixed in with uncirculated coins. For example, a dealer is selling a roll of "uncirculated" coins but one of them is not uncirculated. Therefore the dealer "slid" one in with the rest. The reason others have posted a slider is a coin that is near uncirculated is cause normally the dealer would put one (or more) that is not too dissimilar so that it doesn't stand out. Technically though, a slider can be a very worn coin, but again, it would stand out and call attention to the dealers little trick. So basically, the term has now evolved into any coin that is near uncirculated. As though it could be used as a slider.