pen 2 
(pĕn)
(pĕn)n.
1.
a. A fenced enclosure for animals.
b. The animals kept in such an enclosure.
c. Any of various enclosures, such as a bullpen or playpen, used for a variety of purposes.
2. A roofed dock for submarines.
tr.v. penned or pent (pĕnt), pen·ning, pens
To confine in or as if in a pen. See Synonyms at enclose.
[Middle English, from Old English penn.]