Examples from the Web for underwool
Its hair is more like that of a goat than a sheep, of a brownish gray colour, and with a dense coat of underwool.The Backwoodsman|Various
The underwool is short and soft, as is also the top hair, which is of very pale grey mixed with some yellowish-white hair.
The underwool is short, close and even, as is also the top hair, which is very strong.
The fur is a yellowish brown and rather harsh and brittle and has no underwool.
The underwool is short and rather weak, but regular, as is also the top hair; the colour is usually yellow.