from _http://www.brainydictionary.com/words/po/pound205513.html_
(http://www.brainydictionary.com/words/po/pound205513.html)
and _http://machaut.uchicago.edu/cgi-bin/WEBSTER.sh?WORD=pound_
(http://machaut.uchicago.edu/cgi-bin/WEBSTER.sh?WORD=pound)

pound:  An inclosure, maintained  by public authority, in which cattle or
other animals are confined when taken in  trespassing, or when going at large in
violation of law; a pinfold.

For what it's worth, a quick search doesn't show any usage  of "pound" as a
jail for humans, although one can see how it might be.  The  use of "head" in
the document in question would suggest it was for loose  livestock.  The use of
"key" raises the possibility of it being human, but  perhaps it was simply an
open enclosure with a  lock

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