Oh, yes, I learned it that way in school. Took me until just a few years ago to drop the apostrophe.
It’s an abbreviation of All Hallows’ Eve, “eve” as in Christmas eve and expressions like “the eve of the election”—i.e., the night before. “All Hallows” refers to the following two days, November 1 and 2, traditionally All Saints’ Day and All Souls’ Day, respectively—together, All Hallows—the holy or consecrated.
The idea of the Hallowe’en customs begins with the notion that evil spirits would fly abroad on that night before two holy days, when they would have to lie low. Masks and disguises protect you from them somehow. And you propitiate them with gifts, or else they might play foul tricks on you.
Unless I’m editing a technical manual, I write dialogue and catalogue Also though and through. I never use UR even in texts, and I cringe at it in emails. Some kinds of shorthand are fine; I use abbreviations all the time, esp on groc lists. Other people have their sticking points; for myself, it’s the words.