I've been wondering if the friends he plays with use that word and so it became top of mind for him in that stressful moment. Or worse yet, he picked it up from his Dad. Hope not, in both cases. But if it is the case what does that say about his friends or his Dad. Just conjecturing here. It had to come from somewhere.
Then he needs to change that. And do it now.
I say "Panther Piss!" as my interjection of choice when I'm angry. There's very little chance I'm going to offend a panther.
That's just what he says when he gets upset is a terrible excuse... I mean really TERRIBLE. What if he said "Oh, go beat your wife!" When ever he got upset? Or "Kill a (insert racial slur here)!" When ever he gets upset. It's ridiculous to think "Oh that's just what he says when he's upset."
Interesting. From what I understand you are a "good" (may be even great) putter; so I am guessing for the most part you are probably "target" focused also known as external attention. I can remember playing in an interclub match at Aronimink country club; it was spring, so cool temps, and rained the whole time - I was miserable but managed to win my match. I putted lights out that day - the only thing I remembered is walking up to the ball and focusing on the hole - didn't worry about reading the putt or anything - I wanted to get off the course.