Brand-new means completely new and never used. A brand-new car.
The brand in it is a burning brand, from metal fresh from the fire. Brand-new is one of the very few compounds where the first half no longer means anything on its own.
1adjectivebrand-newCompletely new and never used.
“The brand-new machine failed in the first week.”
brand-new is a quality.
brand new comes from brand + new, implying something that is newly forged (first citation 1570; compare fire-new (“fresh from the fire, brand-new”)). Alternatively, and less likely from brand as in a branding iron (i.e. newly-branded). The first element of the variant bran new, with the post nasal stop deletion common to English (compare the common pronunciation (outside Britain) of hunting as hunning), is claimed to be from bran as if from cases where new items were supposedly “packaged up with unwanted grain (bran) in the 18th century to protect the objects during transit” (source unknown). Both variants are well attested.
Etymology from Wiktionary, CC BY-SA