late 13c., Anglo-French, "to turn over to another," from Old French atorner "to turn, turn to, assign, attribute, dispose," from a- "to" (see ad-) + tourner "to turn," from Latin tornare "to turn on a lathe," from tornus "lathe," from Greek tornos "lathe, tool for drawing circles" (from PIE root *tere- (1) "to rub, turn"). In feudal law, "to transfer homage or allegiance to another lord."
early 14c. (mid-13c. in Anglo-Latin), "one appointed by another to act in his place," from Old French atorné "(one) appointed," past participle of aturner "to decree, assign, appoint," from atorner "to assign," literally "to turn to" (see attorn). The sense is of "one appointed to represent another's interests."
In English law, a private attorney (attorney in fact) was one appointed to act for another in business or legal affairs (usually for pay); an attorney at law or public attorney was a qualified legal agent in the courts of Common Law who prepared the cases for a barrister, who pleaded them (the equivalent of a solicitor in Chancery). So much a term of contempt in England that it was abolished by the Judicature Act of 1873 and merged with solicitor.
Johnson observed that "he did not care to speak ill of any man behind his back, but he believed the gentleman was an attorney." [Boswell]
In U.S., barrister is not used and the general designation became properly attorney and counselor at law; when presenting a case in court, simply counselor. The double -t- is a mistaken 15c. attempt to restore a non-existent Latin original, perhaps by influence of legal Latin form attornare.
*terə-, Proto-Indo-European root meaning "to rub, turn," with derivatives referring to twisting, also to boring, drilling, piercing; and to the rubbing of cereal grain to remove the husks, and thus to threshing.
It forms all or part of: atresia; attorn; attorney; attrition; contour; contrite; detour; detriment; diatribe; drill (v.) "bore a hole;" lithotripsy; return; septentrion; thrash; thread; thresh; throw; threshold; trauma; trepan; tribadism; tribology; tribulation; trite; triticale; triturate; trout; trypsin; tryptophan; turn.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit turah "wounded, hurt;" Greek teirein "to rub, rub away;" Latin terere "to rub, thresh, grind, wear away," tornus "turning lathe;" Old Church Slavonic tiro "to rub;" Lithuanian trinu, trinti "to rub," Old Irish tarathar "borer," Welsh taraw "to strike."