Okay, James and Lily lived in the 70's.
Lily was Muggleborn.
James was Pureblood.
Lily most likely knew the modern slang for that decade.
She probably used it a lot in fact, to rub it in Severus's face she was a Muggleborn.
Imagine James overhearing her say something, like maybe, "Far Out", or "Groovy Man."
He would be so confused.
He worked up the courage one day to ask her what the phrases meant.
She just laughed and walked away.
That was when James realized it wasn't just some little crush on a pretty girl.
He loved her.
