"name the letters of," Old English spellian "to tell, speak," infl. by Old French espeller "declare, spell," from Frankish *spellon "to tell;" both Old English and Frankish from Proto-Germanic *spellan (cf. Old High German spellon "to tell," Old Norse spjalla, Gothic spillon "to talk, tell"), from PIE *spel- "to say aloud, recite." Related: Spelled; spelling.
Meaning "write or say the letters of a word" is c.1400, from notion of "read letter by letter, read with difficulty" (c.1300). Spell out "explain step-by-step" is first recorded 1940, American English. Spelling bee is from 1878 (earlier simply spelling, 1860).
"work in place of (another)," Old English spelian "to take the place of," related to gespelia "substitute," of uncertain origin. Perhaps related to spilian "to play" (see spiel). Related: Spelled; spelling. The noun meaning "indefinite period of time" first recorded 1706.
"incantation, charm," Old English spell "story, speech," from Proto-Germanic *spellan (cf. Old Norse spjall, Old High German spel, Gothic spill "report, discourse, tale;" German Beispiel "example;" see spell (v.1)). Meaning "set of words with magical powers, incantation, charm" first recorded 1570s.
The term 'spell' is generally used for magical procedures which cause harm, or force people to do something against their will -- unlike charms for healing, protection, etc. ["Oxford Dictionary of English Folklore"]