Source : Webster's Revised Unabridged Dictionary (1913)

Overpressure \O"ver*pres"sure\, n.
   Excessive pressure or urging. --London Athen[ae]um.

Source : WordNet®

overpressure
     n : a transient air pressure greater than the surrounding
         atmospheric pressure; "the overpressure of the blast
         kills by lethal concussion"