Shamrock learned it from his papa. Anyways, Oda’s goal is to make Garling and Shamrock look like scumbags.
At this point I think that Mosa has to be either Eris, who was captured and imprisoned in the Holy Land, or Gunko’s subconscious somehow speaking through a den den mushi. Assuming Mosa isn’t just some random dude that Oda will pull out of his ass