As is often the case with lawyers in the United States, they bandy about Latin phrases without any real understanding as to what the Latin words mean, or whether the phrase used as such, really applies to the concept at hand. Latin was the language originally spoken in the region around Rome called Latium. ...

"Ad idem" is a very frequent misspelling of the Latin words ad diem meaning "on a particular day" or "up to the aforementioned day" or "until a particular day" and, if used as an adjective, could even mean "up to date." The reason that lawyers miss this phrase, or misemploy it, is because idem is a legitimate adverb in Latin, and the Norman Invasion that turned the legal world on its head introduced numerous instances of compound conjunctions into the legal language of the time. Thus, the Latin idem (meaning the same) could be compounded with the preposition ad to produce a pair of words employed as a single word, and without anything else to go on, the listener would be at a disadvantage trying to figure out what the lawyer had just said. Bayeux Tapestry depicting events leading to the Battle of Hastings The Norman Conquest was the conquest of England by William the Conqueror (Duke of Normandy), in 1066 at the Battle of Hastings and the subsequent Norman control of England. ...

If the pair of words AD IDEM are actually spelled that way, and are not in fact a misspelling, then it means "to the same" - as in a last will and testament devising real estate to an individual whose identity was painstakingly identified, or if not identified, will be ascertained in short order. AD IDEM, in this sense, has the same function as AD IPSUM. For instance, if a man bequeaths property to a particular John of Blackacre, and a whole paragraph is devoted to the description of this individual, that he not be confused with anyone else, a reference to this same person could be allowed again with a single reference, "AD IDEM" - to the same one - and the will would continue from there.

Another possibility is that AD IDEM is a misspelling for AD ITEM, again a "compound construction" consisting of a preposition and an adverb, something that very likely entered into the legal language of lawyers through and subsequent to the Norman Invasion. AD ITEM means "to the point as well" or "to the same conclusion." AD ITEM is, in turn, a frequent misspelling of ad litem as in "custos ad litem" - as a guardian for the duration of the trial. Ad litem is a term used in law to refer to a party appointed by a court to act in a lawsuit on behalf of another party – for instance, a child or an incapacitated adult – who is deemed incapable of representing themselves. ...