The story
In Old English, a spell was a story, a saying, a tale, a piece of talk. To spell was to speak or tell. Nothing magical about it, at first.
By the 1570s, the noun had picked up a new sense: a set of words believed to have special power, a charm or incantation. A spell was still words, just words that were meant to change things.
The verb took its own road. The meaning “to form words with the right letters” is recorded from the 1580s. It came into English partly through Old French espeller, which itself came from a Germanic word for speaking, and it blended with the native English spellian, “to tell, to talk.” To spell a word is to tell it out, one letter at a time.
Spell it out, meaning explain something step by step, is a much younger American phrase, recorded from 1940. It keeps the old idea alive: make every part clear, as if telling it letter by letter.
