the fine, soft, thick, hairy coat under the longer and coarser outer hair in certain animals, as seals, otters, and beavers.
Also called underwool.

1875–80; under- + fur

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World English Dictionary
underfur (ˈʌndəˌfɜː)
Also called: undercoat the layer of dense soft fur occurring beneath the outer coarser fur in certain mammals, such as the otter and seal

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Example sentences
The pelt consists of soft, thick underfur with long, glossy dark-red to
  dusky-brown guard hairs.
During the winter, their coat is replaced by white fur, but the hair is dusky
  at the base with a gray underfur.
The underfur next to the muskrat's body traps air and prevents its skin from
  getting wet.
Some individuals may have underfur tipped in brown, and these animals will have
  a brownish cast rather than gray.
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