Holy "Ghost" sounds scary, sneaky, harmful- like Christianity. I do use the term "Holy Spirit" to explain the Universe/ intuition/ still, small voice that keeps me out of trouble.

Just another Christian phrase bastardized to instill fear.

The Holy Ghost is all that was left after the Holy Moly died.

Seriously, if you ask father McMahon, he won't be able to tell you, either. Similar to Herman Hesse's remark that the concept of god is an insanity. I think the concept of the Holy Ghost ranks right up there and anyone who argues needs a meeting with the Holy Hand grenade to straighten them out. Please notice the cynical humor.

the holy ghost haunts churches, and the minds of those that go to them ...

I went to a Catholic elementary school and I don't fucking know. They now call it Holy Spirit. A lot easier to believe I guess.

Theologically ... the Holy Spirit (Holy Ghost is Elizabethan English for the same thing, from the King James Bible mostly) is known as "the Comforter", his role is to indwell believers and provide them with insight, wisdom, comfort, counsel, inspiration and the like. Also he's supposed to "convict" you of sin -- in other words he becomes your conscience (presumably, a far more effective and elevated conscience than unbelievers have, although I've seen zero evidence of that). Upon conversion, this person of the godhead is supposed to enter you and take up residence. How this happens and what it means is rather vague and variable.

Vague because although Christians are supposed to be "sanctified" (set apart for god) and improved by this mechanism, the NT also teaches that Christians, although they have a "new nature" and are "a new creation" also struggle with the old "sin nature". So even as a Christian I never saw the point of the Holy Spook. I accepted it in theory, but never sensed its presence, nor did I see evidence of it in others.

Variable because most Christians just teach this as a mystical abstraction but some believe that the Holy Spirit is evidenced by working miracles through you. Charismatics and Pentecostals teach that "speaking in tongues" (glossalalia) is evidence of a separate experience from conversion called the "Baptism of the Holy Spirit" that happens sometime after conversion -- if you seek it earnestly enough, have enough faith, are submitted sufficiently to god, etc. Really spiritual Christians, it's claimed, can additionally have deep insight into situations and the will of god, can heal the sick, and even, some say, raise the dead. As well as do other useless stuff besides speaking in tongues, like getting "slain in the spirit" (fainting on command, basically), or the "gift of prophecy" (wherein one is possessed by the Holy Spirit to speak the words of god himself to the congregation).

That's more than you probably wanted to know, but there you have it ... my personal view is that this doctrine exists mostly to remind believers that god watches everything they do and they should be careful not to offend him (or, worse, piss him off).

I’ve long wondered the same ..though quickly chucked the concept onto the heap of religious ridiculousness

The nuns always told us it was a mystery

God sends him out to haunt you !