It's the song you share that nobody else gives a shit about.
The one where you both know all the words to the rap in the middle, or you make the "widdle widdle" keyboard sound in the middle. Maybe you have a dance to it. Maybe you just finger-point.
Its the song that really should, by all rights, have been your first dance because it belongs to you two, but in the same way you own your stinky cat and your inefficient, overcomplicated microwave, nobody else gets it like you guys do.
If you'd had it as your first dance? The whole room would have gone "WHAT THE FUCK IS THIS?"
So you didn't.
It might be a TV theme, or an advert, a B-side, a novelty hit, or just some weird album track from a record you've listened to to death. Maybe it's something really obvious on the radio that you actually think is pretty terrible, but now it means something to you.
Tell me - what's your secret song?
And do you remember ours?
It's still a HUGE tune. HUGE.