2004 Lewis And Clark Jefferson Nickel Error.

to the CCF.

As already stated, likely struck through 'grease' which is a generic description for several materials that can be inadvertently introduced in the coining chamber and will prevent the coins details from being fully struck up - it does not necessarily mean actual grease like you would use to lubricate say a bearing.

Mike Diamond has weighed in so that all I need to concur.

It's a nice find with some numismatic premium due to the extent of the grease obscured details.