Noun
runt of the litter (plural runts of the litter)
(idiomatic) the smallest and weakest person in a group, especially a group of siblings
As my mum still candidly says, I was the runt of the litter. Nick Moran
I loved performing; I was always trying to impress my siblings by being a clown. I think that came from being the runt of the litter. Rose Byrne