The term 'Sickening Thing' can be broken down into two crossword solutions: TOXIN (5 letters) and GERM (4 letters). A TOXIN is a poisonous substance produced by living organisms, such as bacteria, fungi, and plants. It can cause serious illness or death if ingested or inhaled. A GERM is a microorganism, such as a virus or bacteria, that can cause disease. Both toxins and germs can cause sickness and are considered to be 'sickening things'.