We’re a geeky family, we are. In fact, we start the geeky indoctrination from day one.
So, it really shouldn’t come as any surprise that Adam has always known that if he ever owned a white car he’d call it Shadowfax. And just to help out you non-geeky types I’ll tell you who Shadowfax is. Shadowfax is the lord of horses. He makes his first appearance in The Two Towers. He’s the big, beautiful white horse that Gandalf rides {see clip below}.
Yup, these bad guys, known as Ringwraiths, Nazgûl, Black Riders, Dark Riders, the Nine, The Backstreet Boys, etc. have some mean-looking black mounts. And the baddest of these bad guys is this fellow right here:
Do not piss this guy off. He will stab you. He will stab you with a Morgul-blade. You do not mess with the Witch-king of Angmar.
And since this dude rides a black horse, and we don’t know what his horse’s name* is, we used part of the dude’s “name” instead. And thus, our car was named Angmar.
But, just FYI, Angmar won’t stab you. He might pee coolant on you. But he won’t stab you.

*He probably would have named him a nice, friendly name like ‘Bill’ if somebody hadn’t already taken that name. I’m looking at you Samwise Gamgee!