The smallest and weakest animal of a group born at the same time to the same mother. (Cambridge Dictionary)
A small or weak person who you dislike. (Cambridge Dictionary)
The runt of a group of animals born to the same mother at the same time is the smallest and weakest of them. (Collins Dictionary)
You call a small person a runt when you are annoyed with them and are expressing your dislike for them. (Collins Dictionary)
An animal unusually small of its kind. (Merriam – Webster)
A small pig or other animal, especially the smallest in a litter. (Oxford Dictionaries)
What Is The Runt Of The Litter ?
Runt Of The Litter – What to expect and how to care for runt puppies.