Source : Webster's Revised Unabridged Dictionary (1913)

A fortiori \A for`ti*o"ri\ [L.] (Logic & Math.)
   With stronger reason.

Source : WordNet®

a fortiori
     adv : for a still stronger, more certain reason; "if you are wrong
           then, a foritori, so am I" [syn: {with greater reason},
            {with more reason}]