A friend, who is originally from the UK and not a lawyer, asked me why we call our trainees articling students.
The articling element comes from the articles (provisions, clauses) of the agreement by which a mediaeval apprentice was bound to his (it would have been his in the Middle Ages) principal.
In full it was always articles of clerkship for would-be lawyers, clerk being an old word for anything kind of scribe-y. My late, very old-school father would refer to articled clerks (and pronounce the second word to rhyme with larks not lurks).
In British Columbia, the technical term is articled student; in Ontario and other provinces, it is the rather unlovely articling student, which I suspect dates from the 1960s.
Both come from the verb to article, meaning to be in a formal legal apprenticeship under the supervision of a lawyer.
In England, trainee accountants also article – and there, one is generally articled to someone specific. In Canada, one just articles in a general way, as the first Canadian usage listed in OED suggests: He articled as a student-at-law in September of 1912 (from the Manitoba Free Press, 1918).
The at-law bit is an old fashioned way of saying ‘in relation to the activity of law’; one is also at the bar or a barrister-at-law once called to the bar.
The bar being the railing or other physical barrier surrounding the judge in a courtroom, not one of the cocktail variety – although lawyers are often no strangers to those. By extension, the practice of advocacy itself.
Most of this would baffle an American legal practitioner, who typically does a brief stint as a summer associate (what we would call a summer student), then writes the bar exam and immediately becomes an attorney (in a firm, with the tile of associate, as here).