I just contacted Falk Liebezeit, regional historian from Diepholz, Germany. The Torbeck family from the town of Diepholz was originally named Thor Beek in church records. Falk said that Thor Beek meant 'to the creek' or 'from the creek' in Low German or Plattdeutsch. He expects that there is a farm in the Diepholz region that was once named Beek, but research will be required to verify.
Perhaps not all Torbecks originated from Diepholz, so there are other possible surname definitions from other regions.