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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.
transient acantholytic dermatosis
[origin uncertain; perhaps a shortening of tadpole; perhaps fr British dialect tadde, ''toad'']