Primmin was introduced on S1E8 "The Passenger."
Lt. Toast was introduced on GG:DS9 on Ep 186 "A Choice of Incompetence" at time code 15:50.
Lt. Commander Michael Eddington has taken on the mantle of Lieutenant Toast as of Season 3 Episode 1, :The Search". The drop occurs in the episode "Perplexing Volume".
The nickname comes from the word "Milquetoast"