[oh_sigh]

Except it doesn't! In fact, first cousin once removed(FCOR) can mean both: your parent's sibling's grandchild, OR your grandparent's sibling's child. These are usually two very different relationships(if they exist) - the former FCOR is usually much younger than you, like a niece/nephew, and the latter FCOR is usually much older than you, and more like an aunt/uncle

I wager most native English speakers understand the concept of cousin, but not first/second/etc cousins, nor once/twice/etc removed.