Afterthought

wiktionary

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From after- +‎ thought, probably modelled on forethought. Compare also the verb afterthink.

afterthought

etymonline

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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.