The word "runtiest" is an adjective used to describe something that is the smallest or weakest of its kind. Synonyms for "runtiest" include puniest, weakest, tiniest, and smallest. These words are often used to describe animals or plants that are smaller or weaker than others in their litter or group. For example, a runtiest puppy in a litter may be the smallest and weakest of all the siblings. Similarly, the runtiest plant in a garden may be the smallest and struggling to survive. Other synonyms for "runtiest" may include feeblest, flimsiest, and frailest.