a boy raised like one's own son, though not such by birth or adoption.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2024
How to use foster son in a sentence
Send thou to thy foster-son Harald, and bid him receive from thee the lands and fiefs which they had aforetime here in Denmark.
Then answered the King that it would be called of foul intent to betray his foster-son.
Moreover Harald Eirikson is my foster-son and hath sat on my knee.
There, too, was Thorhall Asgrim's son, and Njal's foster-son.The story of Burnt Njal | Anonymous
Torquil, whose eye never quitted his foster son, saw his emotion, and looked anxiously around to discover the cause.The Fair Maid of Perth | Sir Walter Scott