WordNet (r) 3.0 (2006):

borough English
    n 1: a former English custom by which the youngest son inherited
         land to the exclusion of his older brothers

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

BOROUGH ENGLISH, English law. This, as the name imports, relates exclusively
to the English law.
     2. It is a custom, in many ancient boroughs, by which the youngest son
succeeds to the burgage tenement on the death of the father. 2 Bl. Com. 83.
     3. In some parts of France, there was a custom by which the youngest
son was entitled to an advantage over the other children in the estate of
their father. iller. Rep. mot Mainete.