Is the hairy skin below a chestnut’s tough skin poisonous?

There are two reasons chestnuts are usually eaten fully peeled:

1. The thin fuzzy membrane can be bitter, especially in raw nuts. Cooking and roasting mellows it a bit.
2. The membrane is hard, fibrous and unpleasant, especially compared with the soft interior.

Note that the skin is not poisonous, but depending on the intended use¹ of the chestnuts, it’s probably a good idea to really take the time to clean the nuts properly. If prepared right and still hot, the skin should come off easily.

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¹ Think purée, desserts or marrons glacés. If I buy roasted chestnuts on the street, I just peel as good as I can and don’t bother with tiny pieces of skin.