Why do we say 'brand new'? Where did the 'brand' come from?

Answer 1

The original sense of brand is 'a burning piece of wood in or just taken from a fire'; other senses (e.g., 'a mark made by burning with a hot iron'; 'an identifying mark (of any sort)'; and 'a distinctive kind, class, etc.') all derive from this.

The expression brand-new literally means 'new as if fresh from the fire'. It is first found in the late sixteenth century. A semantically similar expression is fire-new, which is found in Shakespeare ("Your fire-new stamp of honor is scarce current"--Richard III) and elsewhere from the sixteenth century onwards.

Since someone is bound to ask about the phrase brand spanking new: This spanking, which is of uncertain though possibly Scandinavian origin, is a seventeenth-century word with such meanings as 'quick; vigorous'; 'unusually fine, large, striking, etc.'; and as an adverb 'exceptionally; strikingly'. There is also a much earlier (thirteenth-century) expression span-new (also, in combination with our phrase, brand-span-new), where this span is a word of Old Norse origin meaning 'a chip of wood'; span-new is now chiefly dialectal.

Answer 2

The above is one theory - another is that the item in question is so new that you can still read the label/tag/whatever and see who made it - in other words, what brand it is.

The first is more likely the real origin, but this one makes sense too.