The phrase "of the lunatic fringe" is often used to describe people or ideas that are considered extreme, irrational, or outside of mainstream thinking. Synonyms for this phrase include "offbeat," "oddball," "eccentric," "radical," "fanatical," and "unconventional." Other possible synonyms might include "outlandish," "kooky," "quirky," "off-center," "unusual," and "weird." Choosing the right synonym will depend on the context and tone of the situation, but each of these synonyms could effectively convey the idea that someone or something is on the fringes of what is generally accepted as normal or socially acceptable behavior.