Listen – I love a good euphemism as much as the next Shakespeare geek and I can totally see how Robin could be a nice bit of slang for penis. That makes nice work of Robin Goodfellow and Robin Starveling and (other Robins in Shakespeare) but in this case I am disinclined to believe that Ophelia is making a dick joke. This is not because I wouldn’t put it there in her “out the maid never departed more” scene – because that’s in there, for sure. But not now.
Not now because Laertes’ response to this line is to say that she’s turning madness and hell itself into prettiness. Laertes is impetuous but he’s not stupid, nor is he someone who would seem able to ignore an obvious dirty joke.
I suspect that Robin is a name like Dick. It can make you giggle in the right context but sometimes it’s just someone’s name in a song. Or a small cherished bird. Which is definitely not a euphemism. Ahem.