Tad

google

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late 19th century (denoting a small child): origin uncertain, perhaps from tadpole. The current usage dates from the 1940s.

wiktionary

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1877, ‘young or small child’, probably a shortened form of tadpole. The extended meaning ‘small amount’ is first recorded 1915.

etymonline

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tad (n.)

1877, "young or small child," probably a shortened form of tadpole, which is said to be the source of Tad as the nickname of U.S. President Lincoln's son Thomas (1853–1871). The extended meaning "small amount" is first recorded 1915.