A few notes from that link:
1. A strip of type metal, less than type-high and thicker than a lead, used for spacing.
2. A line of cast type in a single strip of metal.
3. A compositor's type line of identifying marks or instructions, inserted temporarily in copy.
tr.v. slugged, slug·ging, slugs
1. Printing. To add slugs to.
if you look at the composition of a WP slug, it involves replacing the spaces of the title with hyphens (when using the default abilities of WP to create the slug for you)
The dictionary is your friend... ;O)