"go weak in my knees"

I used to wonder what one tried to convey by using the phrase. And lo I did experienced what this phrase means. I mean, I literally did go weak in my knees when I saw this guy face to face. He stays on the third floor of my building. I usually see him when he comes to take his bike. But I've never felt this way earlier. Today morning I was leaving for work and he too was leaving for his, dressed in a navy blue blazer and blue trouser. I was left gaping at him. And then try how much ever I may, I was finding it hard to move my feet. It took me a couple of moments to get back my nerve and move on. Well... what do I say about the person who phrased it. Genius :)