A long, thin, regular jack knife which has a flat, one-piece, “seal cap” covering the entire head-end of the knife.  The sealed end was used by old-time physicians as an emergency pestle to grind up pills, making them easier to swallow when mixed with water.  If the bolsters, liners, and springs are exposed at the knife’s head the medicine could get impacted between those parts and/or oil from the knife could mix into the medication.  Some doctor’s knives also have a folding spatula as one of the blades.  A long, slim, jack without a seal cap is a “slim jack.”