Webster's Revised Unabridged Dictionary · 1913

Firstling

Firstling (fẽrst"ling) , noun

[First + -ling.]

1.
The first produce or offspring; -- said of animals, especially domestic animals; as, the firstlings of his flock. — Milton
2.
The thing first thought or done.
The very firstlings of my heart shall be The firstlings of my hand. — Shakespeare

Firstling , adjective

Firstborn.
All the firstling males. — Deut. xv. 19