EXTREME BITTERNESS can be described by two words: VENOM and BILE. VENOM is a toxic substance produced by certain animals, such as snakes, spiders, and scorpions, which can cause serious harm to humans and other animals. BILE is a bitter-tasting, yellow-green fluid produced by the liver and stored in the gallbladder, which helps with digestion and absorption of fats. Both VENOM and BILE are associated with EXTREME BITTERNESS, making them fitting crossword solutions for the term.