Scathingly sarcastic is an expression used to describe a type of humor that is particularly biting and caustic. The crossword solutions for this term are ACID, with 4 letters, and MORDANT, with 7 letters. ACID is defined as a substance with a sour taste and corrosive properties, while MORDANT is defined as something that is cutting or caustic in nature. Both words can be used to describe the type of humor associated with the term 'scathingly sarcastic'.