What is the etymology of the word "spell" when used to mean a short period of time?

Every now and then - usually when talking with an older individual or someone from the United States Midwest or South - I hear the word "spell" used to mean a short period of time, such as: "Come sit down and rest a spell." I've also heard it used as a verb, like, "When Pete arrives he will spell Tito, who has been working since dawn."

What is the history and origins of this meaning and use of the word spell? And is it a colloquialism or can it be used in more formal settings?

Solution 1:

Merriam-Webster gives the origin of this meaning as "probably alteration of Middle English spale substitute, from Old English spala."

The Online Etymology Dictionary says the first recorded use of spell to mean an "indefinite period of time" is in 1706.

Also, M-W does not list this usage of spell as a colloquialism or slang, so I think it's appropriate for formal writing, wherever it is appropriate to use such a vague word.

Solution 2:

To answer the latter part of your question "When Pete arrives he will spell Tito, who has been working since dawn."

Spell: "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.

From here

Solution 3:

Not sure if it's related, but putting a horse out to pasture for a break from work (for weeks, months) is referred to as a spell or spelling.