So, did he stab him?

Now I do realize that we do not know OP's age and I mentioned 'teen'. My comment still applies though. There are plenty of battered and abused women out there and having a family member or good friend that will try to protect you is always a plus. OP you are important to your father and he is trying to get his point across in a manly way.

Maybe it's funny to you because you aren't on the receiving end of it. What if your daughter dated my son and I threatened her with a knife? Would you find it "rather funny" then? Probably not. The actions you described of your partner (forbidding your daughter's relationship) were fine. There is nothing illegal about doing this, as long as your children are under your roof they are your responsibility. Most of the time it is much better to calmly and rationally list the reasons why you think your child should end their relationship and hope they see reason in the end (it may take a while, because as you said "love is blind"). But if a parent decides it is better to overrule and forbid the relationship, I have no issue with this, and it may be the right call. If their child doesn't like it they can move out at eighteen.

what ever happened to the old school dad sitting in a chair with a shotgun?

Ugh NO. This is literally the worst thing you can do as a father. This is why all those "let me get my shotgun" jokes piss me off as well, because they may just be jokes but clearly it's a thing that actually happens and they just seem to make it sound like that's okay. This is the reason I never told my Dad I was with anybody and didn't introduce him to one of my partners until I'd left home. Also, to those people saying that sometimes your daughter might make a bad judgement call and you're just looking out for her? The incorrect response is to physically threaten the boyfriend. The right thing to do is sit down with your daughter and talk to her about your concerns. Tell her that you're worried that she can't see some of the red flags you do from an outside perspective. Get her also to explain why she thinks she's making the right choice. It could be that you're right about them being a bad person for her and she just can't see it, or it could be that you've made unfair assumptions. Overall though, you have to respect her choices. You can't 'make' her split up with her boyfriend and trying to frighten him away with threats is just going to make her stop trusting or respecting you.

Today, my girlfriend's dad stabbed me. FML

Again, depending on the threat I could find it funny. If it's like what OP experienced, then yes. I have a 7 year old son as well, who is my baby and I am very protective of. He was the first baby boy in our immediate family with a string of baby girls in front of him. I spoil him and his grandparents spoil him. I realize this is not in his best interest and am trying to put a stop to it. I have said numerous times that I feel sorry for his girlfriend/wife because they will have to deal with me. No, I would never do what OP's father did (I hope) because that's just not me. Now I'm not above bitch slapping her if she really hurt my son but it would have to be an extreme case (and an adult, I don't hit children). Now a father pulling that stunt on his son's girlfriend, sounds kindof weird but hey maybe more effective.

I realized I might not have answered your question clearly in my above comment. Sorry about that. If I was on the receiving end of this and it was my daughter (or son), yes I would still find it funny.

That's awsome

...and this is the reason so many kids don't trust their parents and won't talk to them about their relationships. Personally I'd rather have a kid who felt they could talk openly and honestly to me about any problems they were having with their relationships. A parent doesn't automatically deserve their child's respect, they have to earn that just as you do with any other human being.