The Collaborative International Dictionary of English v.0.48:

Cui bono \Cui` bo"no\ [L.]
   Lit., for whose benefit; incorrectly understood, it came to
   be used in the sense, of what good or use; and hence, (what)
   purpose; object; specif., the ultimate object of life.
   [Webster 1913 Suppl.]

The Devil's Dictionary (1881-1906):

CUI BONO?  [Latin]  What good would that do _me_?