As far as I know, "quarter" means half of half of something.
Why "no quarter" means "no mercy"?
As far as I know, "quarter" means half of half of something.
Why "no quarter" means "no mercy"?
The term arose from the military where the victorious side publicly proclaimed that it will not provide quarter (prison) for captured enemy. Which meant that no mercy will be shown and all prisoners will end up being slaughtered.
Wikipedia has a more elaborate description of the etymology:
The term may originate from an order by the commander of a victorious army that they "will not quarter (house)" captured enemy combatants. Therefore, none can be taken prisoner and all enemy combatants must be killed.
- as well as an alternative etymology.