Dogmudgeon is a word of Roger's invention, which Nancy agrees is a 'good word'.
The Dogmudgeon or Angus is one of the savage Gaels who the Swallows and the Amazons encounter in Great Northern?. Ian refers to him as Old Angus when he refers to himself, old Angus and the shepherd sending them (the invaders) flying when he had seen them disturbing the deer (GN16).
The shore party had pretended to be geologists and they bang one stone on another (GN5). Ian McGinty and Old Angus saw them disturbing the deer and want to ken who put them up to it if they return. According to Ian Deer are like salmon. They come back to the places where they begin their lives, and by shifting the hinds in the breeding season a dishonest man could ruin his neighbour's deer-forest and improve his own without laying hand on a single beast. It was the very meanest of tricks. And to use children for the driving made it meaner (GN16).
The Dogmudgeon said that they were driving our hinds (GN22). Angus a big grey-bearded man captures Dick at the loch (GN23). Hence Susan says (in the dungeon or barn) that they were thought to be chasing their deer. Captain Flint says that Galloping about on someone else's land doesn't matter much. But chasing deer's serious (GN24). The McGinty says that the chasing of hinds out of their breeding grounds is beyond right and reason (GN26).
But the two shots from the loch change everything. So The McGinty and Captain Flint with the old Dogmudgeon go to confront Mr Jemmerling and his sailor at the loch (GN26,28).