I’ve always vaguely wondered about the phrase past master—was there or wasn’t there also a passed master, and did the one come from the other?—and I’ve finally looked it up in the OED. The earliest form is
pass, v. 40. b. intr. To reach the required standard in an examination, course, etc. Formerly freq. with complement (esp. in to pass master): to graduate as, to become qualified as; (occas.) trans., to approve (a person) as. First cite: ?1566-7 G. BUCHANAN Opinion Reformation Univ. St. Andros in Vernacular Writings 13 Ane of profession of medicine passit maister, and ane regent in humanite.
From there we get passed master as a noun phrase meaning “A person who is especially adept or expert in a specified subject or activity”; first cite: 1882 H. C. MERIVALE Faucit of Balliol I. vi. 96 Faucit was a passed master as a guide to the classics.
But then there’s the more familiar past master, which (it turns out) is originally from the Freemasons:
1. Usu. with capital initial. A person who has previously filled the office of master in a Freemasons’ lodge, civic guild, etc.
1762 Jachin & Boaz 50 The past Master raises him up, and takes off the Jewel and Ribbon from his own Neck, and puts it on the new Master. 1786 Laws Soc. Royal Arch Masons 15 That the three Principals, and all Past-masters are stiled, most excellent. c1826 W. MORGAN Illustr. Masonry (?1851) 80 The second part of this work, which will comprise the following degrees, viz: Mark-Master, Present or Past-Master, Most Excellent Master, and the Royal Arch. [...] 2002 Bristol Evening Post (Nexis) 30 Aug., A Circle compromises the past masters of all Masonic Lodges within the Province of Bristol in any calendar year.
2. A person who is especially adept or expert in a specified subject or activity. With in, of, at.
This use may have arisen partly in allusion to the expertise which results from having passed through such an office as master of a Freemasons’ lodge, etc. Sometimes it simply alludes to the expertise resulting from having passed the necessary training to qualify as ‘master’ in any art, science, or occupation. Cf. PASSED MASTER n.
1840 Southern Lit. Messenger 6 391/1 In our attentions, Mr. Editor, to the Past Masters in Poetry, we are apt to neglect the claims of the entered apprentices of the sublime order. 1872 M. E. BRADDON R. Ainsleigh xv, The man was past-master of all dissimulative arts. 1890 Spectator 13 Sept. 334 A past-master of electioneering tactics. 1936 M. R. ANAND Coolie iii. 164 Munoo had become a past master in the art of slipping by the irregular pedestrians of the city of Daulatpur. 1963 S. BEDFORD Favourite of Gods (1984) 55 Even the prince, himself past-master at leaving well alone, found himself outdone. 1992 Independent 10 Mar. 21/6 Harold Macmillan was a past master at the art of manipulating the economy to produce an election boom.
Just another of those unnecessary bits of confusion in which languages abound.