From the album Lucky - Single
This is a song about watching someone move on at a party and realizing you're already preemptively mourning a relationship that hasn't officially ended yet. The speaker keeps insisting the other person has his love while simultaneously staking out what he'll keep for himself, which means he knows it's already over even if he can't say it out loud.
Embarrassed to feel blue / In my Halloween muscle suit / But didn't have the words to tell you
The costume is doing emotional work here. Wearing fake muscles while feeling small and sad is the whole song in three lines. He can't name what's wrong, so he names what he's wearing instead.
And it scared me / To see you having fun for the first time
This is the line that reframes everything. If this is the first time she's having fun, what does that say about the time they spent together? He's not heartbroken she's happy. He's scared because her happiness doesn't include him.
There's nothing worse / Than dancing when you don't wanna / So I tried to stay busy winning at beer pong
He's describing his own situation as if it's a universal truth. She's dancing because she wants to. He's playing beer pong because he needs something to do with his hands. The song won't say who's really stuck doing something they don't want to do.
You have my love / You have me worried / But my jumpshot / Will always be for me
The jumpshot line is him deciding what he gets to keep when this falls apart. Beer pong earlier, basketball now. He's already dividing assets in his head while still claiming she has his love, which makes the whole refrain sound like he's rehearsing self-sufficiency he doesn't feel yet.
The song ends with the jumpshot refrain repeating five times, which is either reassurance or desperation. Either way, it's the sound of someone trying to convince himself he'll be fine by saying the same thing over and over until it feels true. The repetition doesn't resolve anything. It just runs out of breath.