“Spell” in the reading sense comes from Old French espeler—“to read out letter by letter”—rooted in a Germanic verb meaning “to tell or explain.”

Early readers would laboriously sound out words one letter at a time, and from that slow, painstaking process came the figurative extension: to “spell something out” means to break an explanation down into its most basic steps, leaving nothing to inference. The condescending edge the phrase often carries today is a natural inheritance from that image of talking to someone who can’t yet read.

For example,

Henry said to his employee, “Let me spell it out for you: If I catch you leaving work three hours early again, you’re fired.” 😡🗯