A Smurf is an Ultramarine. Many people call them this because; they are bright blue, nobody likes them, they are viewed as honourable followers of rules, they are boring, if you abbbreviate them to SMU then you get the start of the word Smurf.

Papa Smurf: Used by many people, but with two meanings. The Ultramarines Chapter Master, Marneus Calgar or the Ultramarines Primarch, Rouboute Guilliman.