title_body: Are there concrete examples of slang developing within a conlang?
upvoted_answer: Wiktionary has a list of Esperanto internet slang but most of these I've never seen or heard used before (at least in the main esperanto chats on Telegram).
  
  Esperanto slang terms I've heard or used  
  
  
  krokodili - to speak in a language other than esperanto in a group of esperantists. No one really knows where it originated but the most popular story seems to be that it's because crocodiles have big mouths and small brains. "Ne krokodilu" is a very commonly used phrase.
  sal - hi (short for saluton)
  kvf - how are you (abbreviation of kiel vi fartas)
  mdr - lol (multe da ridoj)
  mns - idk (mi ne scias)
  
  
  Interjections
  
  
  ho ve (occasionally just "ve") - oh geez
  (according to this post on Esperanto SE the literal translation is "oh woe" but who says that)
  8, ok, okej - k, ok, okay
  ("ok" is the word for eight in Esperanto, mdr.)
  ho - oh
  jes ja - yes, indeed
  (according to this post on Esperanto SE "ja" can be used in other situations, but I have never heard or seen it used other than after "jes")
  Fek' - f*ck
  Kio la fek - what the f*ck
  (This is an obvious anglicism and considered "grammatically incorrect" by many esperantists, who will tell you it's actually kio feke)
  Volap*ko - "the v word". More offensive than all the offensive words you know combined.