Green’s Dictionary of Slang

1. the youngest child of a family, conceived long after its siblings.

1910		G.B. Shaw Misalliance (1934) 604: Have you ever considered the fact that I was an afterthought? [...] My father was 44 when I was born. My mother was 41. There was twelve years between me and the next eldest. I was unexpected. I was probably unintentional.
1965		G. McInnes Road to Gundagai 48: Terence was the youngest child [...]. (‘I’m a little afterthought.’).

2. attrib. use of sense 1.

1985		Sun. Tel. 3 Mar. 14/8: Christopher, afterthought son of the founder member, [...] is allowed to sit in on the ladies’ discussions [OED].
2001		Sizzlin’ Sixteen 2001 🌐 For her next learning experience, Ambrose tackles series television, playing the afterthought child (she’s 17; her brothers are 30 and 35) in a family of morticians in the upcoming HBO series Six Feet Under.