Etymology
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afterthought (n.)

1660s, "a later thought," from after + thought (n.). As "reflection after an act," 1680s. Sense of "youngest child of a family" (especially one born much later than the others) is by 1902.

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Definitions of afterthought

afterthought (n.)
thinking again about a choice previously made;
Synonyms: reconsideration / second thought / rethink
afterthought (n.)
an addition that was not included in the original plan;
the garage was an afterthought
From wordnet.princeton.edu