mid-15c., prothogol, "prologue;" 1540s, prothogall, "draft of a document, minutes of a transaction or negotiation, original of any writing" (senses now obsolete), from French prothocole (c. 1200, Modern French protocole), from Medieval Latin protocollum "draft," literally "the first sheet of a volume" (on which contents and errata were written), from Greek prōtokollon "first sheet glued onto a manuscript," from prōtos "first" (see proto-) + kolla "glue," a word of uncertain origin.
The sense developed in Medieval Latin and French from "rough draft; original copy of a treaty, etc." to "official record of a transaction," to "diplomatic document" (especially one signed by friendly powers to secure certain ends by peaceful means), and finally, in French, to "formula of diplomatic etiquette." That final sense is attested in English by 1896.
The general sense of "conventional proper conduct" is recorded from 1952. "Protocols of the (Learned) Elders of Zion," Russian anti-Semitic forgery purporting to reveal Jewish plan for world domination, first was published in English 1920 under title "The Jewish Peril."