The Collaborative International Dictionary of English v.0.48:

Undersheriff \Un"der*sher`iff\, n.
   A sheriff's deputy.
   [1913 Webster]

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

UNDER-SHERIFF. A deputy of a sheriff. The principal is called high-sheriff,
and the deputy the under-sheriff. Vide 1 Phil. Ev. Index, h.t.