Stud Duck is the term that Ennis uses to descrobe Jack's father. It is used in the thanksgiving scene by Lureen's father.
Correct.
In her essay which accompanies the screenplay, Proulx mentions that it took her awhile to get the proper term for Jack's father during Ennis' post-mortem visit to Jack's parents.
Of course, this term was never used in the short story as part of the dialog between Ennis and Jack's parents. It is the voice of the narrator of the story who pins it on Jack's father. It is interesting that they lifted this particular word and put it in Lureeen's father's mouth during the Thanksgiving scene in Jack's house.
I guess the screenwriters wanted to incorporate this important term somehow in an actual dialog involving Jack. For Jack's father to have described himself
out-loud as a stud duck in the movie would have sounded strange. I think making Lureen's father vocally use it to refer to himself worked better.