last child; youngest child
Refers to the youngest child in a family, often implying the final child born. Somewhat literary or formal; in casual speech 末っ子 (すえっこ) is more common.
彼は五人兄弟の季子だ。
He is the youngest of five siblings.
季子として甘やかさ
As the last child, I was spoiled growing up.
The kanji 季 means 'season' or 'end of a period', and 子 means 'child'. Together they suggest the child who comes at the end, i.e., the last child. The exact historical derivation is uncertain, but the term is used in literary Japanese.