The Collaborative International Dictionary of English v.0.48:

Pen \Pen\, v. t. [imp. & p. p. Pennedor Pent (?); p. pr. &
   vb. n. Penning.] [OE. pennen, AS. pennan in on-pennan to
   unfasten, prob. from the same source as pin, and orig.
   meaning, to fasten with a peg.See Pin, n. & v.]
   To shut up, as in a pen or cage; to confine in a small
   inclosure or narrow space; to coop up, or shut in; to
   inclose. "Away with her, and pen her up." --Shak.
   [1913 Webster]

         Watching where shepherds pen their flocks at eve.
                                                  --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pent \Pent\ (p[e^]nt), p. p. or a. [From Pen, v. t.]
   Penned or shut up; confined; -- often with up.
   [1913 Webster]

         Here in the body pent.                   --J.
                                                  Montgomery.
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         No pent-up Utica contracts your powers.  --J. M.
                                                  Sewall.
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The Collaborative International Dictionary of English v.0.48:

pent- \pent-\ (p[e^]nt), pref.
   Same as penta-; -- used as a combining form before vowels,
   as in pentoxide.
   [PJC]