Seems fairly self-explanatory to me. Surely it's referring to the white froth at the top of a glass of shandy and making a comparison with spunk (i.e. you're shaking your hand to produce your own "froth"). The reason it's shandy and not say, beer or any other frothy drink is because hand rhymes with shand(y).
No one calls the froth on shandy “shandy”, though. It’s froth.
Have you ever heard a conversation that went:
“What’s that white stuff on top of your drink?”
“Shandy.”
“Yeah, I know you’re
drinking shandy, but what’s the white stuff on top of the shandy?”
“Shandy.”
Well, have you? Have you ever heard that? You haven’t. You haven’t heard that. Know why you haven’t heard it? Because no one calls the froth on shandy “shandy”.