A sheath is a protective covering, and the verb to strip means to remove something from someone. So, if the hazelnuts (or the speaker) are stripped of whatever protects them, then they are defenseless. Much in the way that the hazelnuts have lost their protective covering, the speaker has fallen prostrate and given into the overwhelming power of the orchard.
This “stripping” draws a connection to the violence of “flayed” in the earlier stanza. Stripping may also be read as a metaphor for sexual nudity, comparing the sheaths of the hazelnuts to human clothing. When read this way, the poem may be seen as a lesson in resisting the earthly temptations of sex and desire.
