When someone says that "it borders on insanity," they are describing a situation or behavior that is dangerously close to being considered crazy or irrational. Some synonyms for this phrase include "it is on the brink of madness," "it verges on lunacy," "it teeters on the edge of delirium," and "it approaches the realm of hysteria." All these phrases convey a sense of alarm and concern, as if the person is warning others to be careful or to take action before things spiral out of control. Using these synonyms can add depth and intensity to your descriptions and help you convey strong emotions.