Final answer:
The speaker is threatening to shoot the person with buckshot, causing them to walk like a duck, which signifies a severe and potentially violent action, not a literal lesson on walking like a duck or the gifting of a pet duck.
Step-by-step explanation:
In the line "Well, shet yer face, er I'll fill yer behind so full of buckshot, you'll be walking around like a duck for the rest of yer life," the speaker is threatening to shoot the person with buckshot. This is a vivid and aggressive way to tell someone that if they do not stop talking or do as they are told, the speaker will take severe action, implied to be shooting them in a way that would make them walk awkwardly, like a duck. This is not a literal suggestion that they will teach the person how to walk like a duck, give them a pet duck, or simply tell them to be quiet; it is a metaphorical expression of potentially violent intent.