Being a slang for "having eaten too much bread in one sitting", "painful" is derived from the French word "pain" (meaning "bread") and the English suffix "-ful".
After binge-eating too much bread in one sitting, I'm now feeling painful due to having my stomach full.
EBK stands for "Elite Ball Knowledge." 1 In internet slang, "ball knowledge" originally referred to a deep understanding of football/soccer or basketball, but it has evolved into a broader term for having a highly sophisticated, deep, or niche understanding of any specific topic (such as music, movies, or subcultures).
Person A: "That scene was a direct homage to an unreleased 1982 Japanese indie film."
Person B: "Wow, EBK."