SLANG THAT I DIDN'T UNDERSTAND AND HAD TO URBAN DICTIONARY

I'm a 20 year old who's not with the times. Here are my initial reactions to them.
  1. TL;DR
    Ehhh??? What??? What's the semi-colon there for? Is this a technological term?!
  2. Truffle butter
    IS THIS REGULAR BUTTER INFUSED WITH EXPENSIVE TRUFFLES FROM ITALY OR FRANCE OR WHEREVER??
  3. Stan
    Who's Stan and why is everyone talking about him?
  4. FOMO
    ??????????????????
  5. Lit
    Did someone catch on fire???? 😱😱😱
  6. ICYMI
    Huh???
  7. LRT
    Ummmm... What about a retweet?
  8. Bye Felicia
    Who's Felicia? Where did she come from? Where is she going??
  9. Bonus: FLOTUS and POTUS
    Not slang, but it took me forever to connect the First Lady/President of the United States to these abbreviations 😁