The Collaborative International Dictionary of English v.0.48:

Puer \Pu"er\, n. [Etymol. uncertain.]
   The dung of dogs, used as an alkaline steep in tanning.
   --Simmonds.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PUER. In its enlarged sense this word signifies a child of either sex;
though in its restrained meaning it is applied to a boy only.
     2. A case once arose which turned upon this question, whether a
daughter could take lands under the description of puer, and it was decided
by two judges against one that she was entitled. Dy. 337 b. In another case,
it was ruled the other way. Rob. 33.