Gravitas is seriousness and dignity. You might try to speak with gravitas, but no one will take you seriously if you're wearing clown makeup and giant floppy shoes.
Gravitas is a Latin word that means "weight or heaviness." It came to mean a figurative weight after gravity acquired a primarily scientific meaning. A biography of Abraham Lincoln will inevitably be full of gravitas, and places like libraries, museums, and university buildings seem to have gravitas, or dignity — while grocery stores and gyms usually do not.