Meaning
A wooden coffin with a lid, the box used to bury the dead. It stands for death and funerals, and in spooky contexts for vampires and horror.
In everyday chat it is mostly a dramatic joke about being so tired, embarrassed or amused that you could die: “That video had me crying, I’m in the ⚰️ now.” For real losses, gentler emoji such as 🕯️ or 🕊️ usually fit better.