From the short story, Jack recounts for Ennis his father's crazy rage when he was only three or four years old:
"Christ, he licked the stuffin' out a me, knocked me down on the bathroom floor, whipped me with his belt. I thought he was killin' me."
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Annie Proulx ends her "-ing" words as "-in". I've added an apostrophe to her "stuffin" and her "killin".