"A pun does not commonly justify a blow in return. But if a blow were given for such cause, and death ensued, the jury would be judges both of the facts and of the pun, and might, if the latter were of an aggravated character, return a verdict of justifiable homicide"

The quote is stating that making a pun, or a play on words, ought to not result in physical violence towards the individual who made the pun. However, in the case that somebody did physically damage another individual for making a pun, and it led to their death, a jury would need to choose if the pun was particularly offensive or painful, and if so, they might possibly rule that the violence was justifiable. Oliver Wendell Holmes is making a point about the significance of context and objective in legal matters.