To fall in love.
From the movie Bambi:
"Nearly everybody gets twitterpatted in the springtime. For example, you're walking along minding your own business; you're looking neither to the left, nor to the right when all of a sudden you run smack into a pretty face... Who-o! Who-o! You begin to get weak in the knees, your head's in a whirl! And then you feel light as feather, and before you know it you're walking on air, and then, you know what? You're knocked for a loop! And you completely lose your head!
She makes my heart go twitterpat.
She's got me all twitterpatted.