The short answer

The phrase "brand new" most likely connects back to the idea of something new being created out of a hot furnace, fire, or forge. In Old English, "brand" referred to a burning piece of wood.

The long answer

Arguably, our ability to create fire on a whim is one of the most impactful technologies humans ever figured out. Because fire helps us create things like tools, cooked food, and so much more.

See where this is going?

When we say something is "brand new" we are describing it completely new and unused. The word "brand" dates back to Old English and referred to a burning piece of wood. This etymology implies that something "brand new" is like an item fresh out of the fire, still glowing from the forge's intense heat.

Interesting, the use of "fire-new" was also used around the time and holds the same meaning of newness. Famous playwright William Shakespeare, among others, used it frequently in his plays:

"Brand new," and its variations "bran-new" and "fire-new," all share a common connection of referring to something new being created out of fire.

🧠 Bonus brain points

Why do we say brand "spanking" new?

The concept of something being "brand spanking new" likely tracks back to the Old Norse word "spānn", meaning a "chip of wood." The same idea of newness is captured in a freshly cut wood chip. But interestingly, the use of "spanking" to describe newness predates the verb "spank" by almost 50 years. So the tree meaning came before the slap meaning. The more you know!

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Sources

Kellerman, S., & O’Conner , P. T. (2007, April 10). A “brand-new” question. Grammarphobia. https://www.grammarphobia.com/blog/2007/04/a-brand-new-question.html

Martin, G. (n.d.). The meaning and origin of the expression: Brand spanking new. The Phrase Finder. https://www.phrases.org.uk/meanings/brand-spanking-new.html

Merriam-Webster. (n.d.). The History of “Bran(d)-new.” Merriam-Webster. https://www.merriam-webster.com/wordplay/word-history-brand-new