Behind the Dictionary
Lexicographers Talk About Language
As a teenager, I got the impression that an easy way to make any insult extra-offensive was to say it carelessly, as if you were drunk. But eventually I realized that a slur is not a mumbled remark expressing general disrespect about someone. On the other hand, even the most carefully enunciated insult can qualify as a slur, provided it's grounded in race, religion, or other historical bases for discrimination.
In fact, current usage of slur can be expressed as a two-way condition: Call an insult a slur if, and only if, you want to identify it as hate speech, based in socially unacceptable bias. In the last few news cycles, we've heard too many stories involving slurs, and beyond whatever this may say about our society in general, these stories highlight this meaning restriction on slur. Originally, though, slur didn't have any particular association with these socially charged distinctions.
According to historical dictionaries, slur seems to have multiple origins near the end of the 16th century. One is a verb referring to a gliding or smooth transition, and the other (according to the Oxford English Dictionary) is a now-archaic noun referring to a thin mud (which survives as the related noun slurry). As the noun came to be used as a verb, the concrete meaning of "smear with mud" led to the abstract meaning of disparagement, cited in the OED from 1658. Meanwhile, the "glide" meaning of the verb seems to have developed its own insult-laden sense of "to pass over lightly, without proper mention or consideration." (In point of fact, the OED lists this meaning under the slur relating to stains and smears, not under the gliding-related slur, but I think my connection is at least as plausible.) Finally, laden with its meanings of disparagement, the verb slur crossed back into noun territory. In short, slur with the meaning of an insult was the result of the "verbing" of an archaic noun, followed by the "nouning" of the resulting verb.
In addition to the fraught semantic restriction on slur I mentioned earlier, there's a somewhat duller semantic restriction: A slur these days is a noun. In fact, you can usually identify which noun it is, depending on the particulars of the news story. As recently as 60 years ago, however, it often referred to an entire proposition about someone, whether stated explicitly or just implied. For example, here's a passage from the Corpus of Historical American English, taken from Miss Ravenel's Conversion from Secessions to Loyalty, by J. W. De Forest, published in 1867:
Colburne was not badly pleased with this speech, inasmuch as it seemed to convey a slight slur upon Mr. Whitewood.
Even more striking is to find instances of the phrase racial slur in the 1940s and 1950s in news stories involving African Americans, and discover that they don't refer to the specific noun now often circumlocuted as "the N-word." In an article in The Afro American newspaper of Feb. 10, 1945, an Italian newspaper was lambasted for using a "racial slur," which turned out to be the implied message that black American soldiers were not accustomed to wearing shoes. And then there was this story from August of 1956, about Jackie Robinson, the first black player in modern Major League Baseball. A Milwaukee Braves player was accused of using a "racial slur" against Robinson, but the slur wasn't what you would think: Instead, it was an entire sentence about a watermelon—supposedly a reference to Robinson's expanding waist, but taken by Robinson (possibly correctly) to be a reference to a racial stereotype.
Then as now, racial slur could also refer to a noun instead of a complete sentence. This 1957 article quotes the NAACP as saying that it "certainly objects to any term that denotes [sic] a racial slur," and singling out three in particular. However, it's hard to say when the shift toward near-exclusive noun reference came about.
In any case, racial (or racist) slur really starts to pick up during the Civil Rights Era, as evidenced by its becoming frequent enough in written sources to show up in the Corpus of Historical American English. From the 1960s onward, its numbers in COHA have increased almost every decade. From the 1970s onward, you also get racial/racist insult and racial/racist epithet, but the slur collocation is always in the lead, usually by a lot. Epithet, by the way, is useful in that it for as long as it has been a synonym for insult (since the 1712, according to the OED), it has referred exclusively to nouns, not to entire propositions as slur historically did.
The 1970s are also when ethnic slur makes its COHA debut. Anti-Semitic slur is a bit older. It first appears in COHA in 1962, but Google Books has an earlier example from The Jews in America, by Rufus Learsi, published in 1954. From this core of terms related to racial and religious discrimination, other adjective collocations with slur start trickling in, tracking the expanding societal awareness of other kinds of discrimination. Looking at both COHA and its sister corpus, the Corpus of Contemporary American English, we have religious slur from the 1980s (Judaism being the relevant religion again), and sexist, anti-immigrant, and homophobic slur from the 1990s. These are joined in the 2000s by gay, homosexual, anti-lesbian, anti-vegetarian, anti-disability, and fat slur. In short, slur may have narrowed its range to nouns, but it has expanded its range from racial and religious discrimination to become the word of choice when it comes to deploring any kind of epithets based in bias or bigotry.
The current state of affairs, with slur used almost exclusively to refer to nouns of an insulting character, is creating some changes in the syntax of slur. In listening to some of the recent news stories, the phrase "called him an ethnic slur" caught my attention. You can use a slur and you can write a slur, because a slur in this sense is a word, and those are things you can do with words. But when I heard "called him an ethnic slur," I imagined someone saying, "You're an ethnic slur, you know that?" It reminded me of someone actually pronouncing the words "expletive deleted" in their own speech, or making a beeping noise instead of a taboo word. On the American Dialect Society email list, Ben Zimmer observed that this was clearly an extension of the idiom call someone names. COCA even has attestations of this variant of the idiom, going back to 1993. Arnold Zwicky has written a blog post on this piece of unusual syntax.
The adjective collocates for slur that I've focused on here have all referred to various populations with some characteristic that might be targeted for harassment. However, they don't constitute the full list of adjectives co-occur with slur in its uglier sense. As a closing note, here are some other useful adjectives I found in COCA that can describe a slur: derisive, derogatory, insidious, nasty, dehumanizing, monstrous, infamous, offensive, vile, hateful, intolerable, and evil.