Caustic solutions are chemical compounds that contain a high concentration of hydroxide ions, such as sodium hydroxide or potassium hydroxide. These solutions are highly corrosive and can cause severe burns if they come into contact with skin. The term 'CAUSTIC' comes from the Greek word 'kaustikos, ' which means 'to burn.' The crossword solution for CAUSTIC solutions is 'lyes, ' which is an archaic spelling of 'LIES, ' meaning 'alkaline solutions.'