I always used to mix up Limbo and purgatory. Well, okay, to be honest I still do. But now at least I know that "Limbo" is called that because the unbaptized souls had to bend over backwards as they tried to dance under--
You're not buying it, are you.
Okay. Actually the term derives from Medieval Latin "limbus," which derives from the Latin word for "border."
I'm uncertain, however, whether that word is at all related to Latin "limen," meaning "threshold" (hence "subliminal," et alia), or whether they just happen to share some sounds and related meanings.