Go off the deep end is an idiom that means to become overly emotional or to act in an extreme manner. The crossword solutions SNAP and DIVE both refer to this phrase. SNAP is a verb meaning to make a sudden, sharp sound or to break suddenly and quickly. DIVE is a verb meaning to plunge into water head first. Together, these two words can be used to describe someone who is going off the deep end, as if they are suddenly and quickly plunging into an emotional state.