from Wiktionary, Creative Commons Attribution/Share-Alike License
- v. To release from a pen.
from the GNU version of the Collaborative International Dictionary of English
- transitive v. To release from a pen or from confinement.
from The Century Dictionary and Cyclopedia
- To let out or release from being penned or dammed up; set free from a pen or confinement.
- To deprive of feathers.
un- + pen (Wiktionary)