[f. FOSTER sb.1 Cf. ON. fóstrsonr.] One who is brought up as a son though not one by birth.

1

a. 1450.  Le Morte Arth., 2955. That fals traytor, sr Mordreid, The Kynges foster sone [Furniv., soster son] he was.

2

1697.  Dryden, Virg. Pastorals, IV. 66.

        Mature in Years, to ready Honours move,
O of Cœlestial Seed! O foster Son of Jove!

3

1894.  J. T. Fowler, Adamnan, Introduction, p. lxxxii. He was called the foster-son or alumnus of Adamnan.

4