Someone who creeps around in a group of friends blending in but contributing nothing to the group socially or productively. Most of the time this person is a stand in friend for numbers sake.
"That man has not moved from that seat or said one word all night, what a slug."
"Hey Keisha, how come slugs are always running?"
"Slugs have 4(four) noses , Matt, that's why there always running!
Slugs is the sensation of spit build up in your throat while you are high that have a similarity to if you swallowed slugs
High Man: spit feels weird in my throat bro
High bro: oh you got slugs man