The Collaborative International Dictionary of English v.0.48:

Subtenant \Sub*ten"ant\, n. (Law)
   One who rents a tenement, or land, etc., of one who is also a
   tenant; an undertenant.
   [1913 Webster]

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

SUB-TENANT. The same as under-tenant. See Under-leaser; Under-tenant, and 1
Bell's Com. 76.