The Collaborative International Dictionary of English v.0.48:
A fortiori \A for`ti*o"ri\ [L.] (Logic & Math.) With stronger reason. [1913 Webster]WordNet (r) 3.0 (2006):
a fortiori adv 1: with greater reason; for a still stronger, more certain reason; "if you are wrong then, a fortiori, so am I"