The expression appears in Magna Carta (to the extent that the original is in the Latin of the day).
Every now and then some ratbag tries to assert that he is entitled by Magna Carta to a jury of his peers, which he self-servingly defines. The first problem is that the Magna Carta is now of only historical and not legal interest. Primary school children get taught that it is the basis of freedoms etc, but that is so (to the extent that it even is so) in a historical sense, not a contemporary legal sense. In practice, its provisions have been so diluted, modified and repealed that it is irrelevant. I don’t know of a jurisdiction that derives its jurisprudence historically from England where this is not so.
At one point in history there was a social and legal distinction between members of the nobility and commoners, each of which did not treat the other as “peers”. It was, for example, possible for a member of the House of Lords to insist on a trial by other members of the House, rather than an ordinary jury. It seems that Lord Cardigan (of Charge of the Light Brigade fame) insisted on onesome time before he became historically more famous.
At the present day, however, all citizens are “equals” and those who insist that they are sufficiently special that their jury should only come from some small selected sub-group always lose.
The following passage is taken from a case in my jurisdiction where an argument about who could comprise the jury of a man charged with wilful damage occurred. The accused claimed to be a descendent of an aboriginal tribe, and therefore tried to insist that his jury should come from co-members also so descended. The reference to Australia does not reduce the generality of the extract. In short, peers means “equals” in the sense of “equals before the law”, and that means everyone.
"In popular imagination a reference to Magna Carta invariably means the Great Charter granted by King John at Runnymede in 1215. A recent American commentator Professor Thomas G. Barnes reminds us that Magna Carta is “more often cited than read”. Perhaps that is because it was written in Latin. The special edition published by the Legal Classics Libraryof Birmingham, Alabama, to which these and other remarks by Professor Barnes form an introduction, offers the following translation of ch.39:
“No freeman shall be seized, or imprisoned, or dispossessed, or outlawed, or in any way destroyed; nor will we condemn him, nor will we commit him to prison, excepting by the legal judgment of his peers, or by the law of the land.”
The edition referred to is a reprint of an historical essay by Richard Thomson published in 1829, in which, at p.228, it is suggested that “the
word Peer was probably originally derived of the Latin Par an equal…”, adding that “the trial by equals is of great antiquity…”. To accept that what is required by ch.39 is a trial by “equals” is effectively to dispose of the appellant’s second ground of appeal. For, in contemporary Australia, all individuals are equal before the law, and, whatever else may be said about those who comprised the jury at the trial of the applicant in this case, they were at law certainly all his equals, as he was theirs."