Say it isn't so!' is an expression of shock or disbelief at hearing something unexpected. The crossword solutions for this term are DENY, RECANT, REBUT, NEGATE, OHNO, and IHOPENOT. DENY means to refuse to grant or admit something, RECANT means to take back a statement or opinion, REBUT means to oppose or argue against something, NEGATE means to DENY the existence or truth of something, OHNO is an exclamation of surprise or dismay, and IHOPENOT is a contraction of 'I hope not'.