The Collaborative International Dictionary of English v.0.48:

Tac-au-tac \Tac`-au-tac"\, n. [F., fr. riposter du tac au tac to
   parry (where tac imitates the sound made by the steel).]
   (Fencing)
   The parry which is connected with a riposte; also, a series
   of quick attacks and parries in which neither fencer gains a
   point.
   [Webster 1913 Suppl.]