Here is the truth behind the archaic expression.
When looking at older prayer cards in English, it will often refer to God as the “Holy Ghost.” This was a common way to identify the third person of the Holy Trinity and was an extremely popular term used in prayers and catechism classes that date before the mid-20th century.
What does that mean? Is God some sort of “spooky” spiritual being?
In modern usage the word “ghost” is used almost exclusively as a term for a deceased person who haunts an individual. When someone says the word “ghost,” often the immediate image that pops into a person’s mind is that of a white bedsheet, or a hazy apparition seen in horror movies.
However, the English word “ghost” didn’t always have this association. The word was derived from the Old English word “gast,” which is related to the German word “geist.” Originally these words simply meant “spirit.” For many years Holy Ghost and Holy Spirit were used interchangeably to identify the third person of the Holy Trinity.
Eventually Holy Spirit replaced Holy Ghost, being a preferred translation of the Latin “Spiritus Sanctus.”
In reality, both words are valid ways to describe God, but because of the modern association of “ghost,” it is no longer in wide usage.
The many names of God and what they mean