GR translation: avast you scum

[Mike Jing]

P172.29-36   His is the hale alcoholic style celebrated in national
legend and song. He is descended directly from the Welshman in Henry V
who ran around forcing people to eat his Leek. None of your sedentary
drinkers though. Pointsman has never seen Gwenhidwy off of his feet or
standing still— he fusses endlessly pitch-and-roll avast you scum down
the long rows of sick or dying faces, and even Pointsman has noted
rough love in the minor gestures, changes of breathing and voice.

What is "avast you scum" doing in there?