The term 'caustic stuff' refers to substances that are highly corrosive and can cause severe burns. The crossword solutions for this term are ALKALI (6 letters), lyes (4 letters), LYE (3 letters), and ACID (4 letters). ALKALI is a type of base, which is a substance that has a pH greater than 7. Lyes and LYE are both common names for sodium hydroxide, a strong ALKALI. ACID is a type of substance that has a pH less than 7. All of these substances are caustic and can cause severe burns if they come into contact with skin.