The term 'PEA-BRAIN' is a slang term used to describe someone who is not very intelligent. The crossword solutions for this term are MORON (5 letters) and MENDEL (6 letters). Gregor Mendel was an Austrian scientist who is known as the father of modern genetics. He is famous for his experiments with pea plants, which led to the discovery of the laws of inheritance. The word 'MORON' is a derogatory term used to describe someone who is not very intelligent.