This is an expression that I occasionally run into and whose meaning I can never seem to remember (It's Greek to me! That shows what I know.<2020 “The U.S. is not the U.K. . . . and ‘ Bernie Sanders is not Jeremy Corbyn.” A humorless oddball dogged by persistent charges of anti-Semitism, Corbyn is personally unpopular . . . Corbryn is ‘sui generis’ . . . so is the Brexit morass.”—The Week, 27 December/10 January, page 6>
) and which I find hard to even guess at from the context. So perhaps if I do a posting maybe it will help me and others who may or may not be familiar with it and cement into their noggins. 
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sui generis adjective [[sometimes italicized, sometimes not]]: constituting a class alone, in a class of its own, a thing of its own kind: of his, her, or its own kind, one of a kind, unique, peculiar <possesses certain sui generis qualities>
May be used predicatively or postpositively <the man is sui generis> <a history book sui generis>
Synonyms: alone, lone, one, one-off, singular, sole, solitary, special, only, unique
Etymology: Borrowed from Latin. From suī (of its own) + generis, the genitive of genus (origin, kind, class). Literally meaning “of its own kind/class”.
First Known Use: 1694
(Merriam-Webster Unabridged Dictionary, Dictionary.com, Wiktionary.com)
The following quotes are from The Oxford English Dictionary and archived sources:
_______________________________<1963 “In this theory meaning is defined as a sui generis ‘reciprocal relation between name and sense, which enables them to call up one another’.”—Structural Semantics by J. Lyons, i. page 2>
<1977 “The superlative interpretations by the sui generis Budapest Quartet come from tapes of live performances at the Library of Congress in 1959 and 1961.”—Time Magazine (New York), 4 April, page 41/3>
<1995 “The building, which was completed in 1926, is sui generis. Parts of it are modeled on a 15th Century Italian monastery while the rest combines elements of a number of Renaissance palaces.”—Chicago Tribune (Chicago, Illinois), 23 July, page 166>
<2011 “Now, some people defy comparison. I believe firmly, for example, that Dennis Johnson was completely sui generis. No NBA person ever–ever!–says, ‘this kid reminds me of Dennis Johnson.’”—The Boston Globe (Boston, Massachusetts), 30 January, page C11>
<2016 “Yet in a number of respects Assemble [[an art collective]] is sui generis, suggesting a new and ever-shifting model for socially engaged art practice.”—The Los Angeles Times, 1 May, page F6>
<2018 “Further, I suspect you’ll find few Republican veterans seeking office who would criticize the commander in chief. One notable exception, of course, was McCain – a maverick to the last. He was sui generis and his seat would be hard for anyone to fill.”—Wisconsin State Journal (Madison, Wisconsin), 22 December, page A7>
<2019 “The two dozen essays of ‘Human Relations’ are culled from five decades of work . . . There are essays on the vapidity of novel reviewing . . . and on a sui generis one-volume encyclopedia . . . a volume from which Wilmers teases one astonishing detail after another.”—New York Times Magazine, 27 October, page 33>
Ken Greenwald – February 3, 2020
