The Collaborative International Dictionary of English v.0.48:

arc-boutant \arc`-bou`tant"\ ([aum]r`b[=oo]`t[aum]N), n. [F.]
   (Arch.)
   A buttress that stands apart from the main structure and
   connected to it by an arch; same as flying buttress.
   --Gwilt.

   Syn: flying buttress
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

arc-boutant
    n 1: a buttress that stands apart from the main structure and
         connected to it by an arch [syn: flying buttress, arc-
         boutant]