Haddest notion means an idea that one has. There are many synonyms for this phrase, such as a thought, a concept, an opinion, a hunch, or a belief. Each word has its own nuances and connotations, but they all refer to a mental process of grasping an idea or understanding a situation. Some synonyms may imply a more logical or rational process, such as a theory or a hypothesis, while others may suggest a more intuitive or emotional response, such as a suspicion or a gut feeling. Ultimately, the choice of synonym depends on the context and the tone of the communication, as well as the speaker's personal style and preference.