Republican In Name Only (RINO) is a pejorative term used by conservative members of the United States Republican Party to describe Republicans whose political views or actions they consider insufficiently conservative. The acronym emerged in the 1990s. Prominent Republicans who have been called RINOs include former Illinois Senator Mark Kirk, Maine Senator Susan Collins,  and Vermont Governor Phil Scott.
In 1912, former President Theodore Roosevelt, then-President William Howard Taft, and Senator Robert M. La Follette fought for ideological control of the Republican Party and each denounced the other two as "not really Republican." The phrase Republican in name only emerged as a popular political pejorative in the 1920s, 1950s, and 1980s.
Bill Clinton would have been proud of what was happening on the third-floor Senate corner at the State House this week. ... The Republicans were moving out and the Democrats and "RINOs" (Republicans In Name Only) were moving in.
Buttons featuring the red slash through an image of a rhinoceros were spotted in the New Hampshire State House as early as 1992. In 1993, future California Republican Assembly President Celeste Greig distributed buttons featuring a red slash over the word RINO to express opposition to Los Angeles mayor Richard Riordan. The term came into widespread usage during subsequent election cycles.
That little sobriquet is so baseless and so outrageous ... [it was] ginned up by people who don't believe you can be a Republican unless you're hard right on social issues.
During Republican primary campaign season, some conservative organizations target RINO Republicans who fail to adopt their stances. National Federation of Republican Assemblies started the "RINO Hunters' Club", whom they believe to be too moderate on such issues as taxes, gun rights, and abortion. The fiscally conservative 501(c)4 organization Club for Growth invented the "RINO Watch" list to monitor "Republican office holders around the nation who have advanced egregious anti-growth, anti-freedom or anti-free market policies"; other conservative groups published similar lists.
More recently, the term has been used to describe Republican critics of President Donald Trump, regardless of ideology. These include former Senators Jeff Flake of Arizona and Bob Corker of Tennessee, Utah Senator and 2012 Republican presidential nominee Mitt Romney, Massachusetts Governor Charlie Baker and former Ohio Governor John Kasich. 
While the term RINO is of recent coinage, the concept of being an inauthentic member of the Republican Party by not representing its more conservative faction is a recurring theme in Party history.
In the 1930s and 1940s, Me-too Republicans described those running on a platform of agreeing with the Democratic Party, proclaiming only minor or moderating philosophical differences. An example is two-time presidential candidate Thomas E. Dewey, who ran against the popular Franklin D. Roosevelt and his successor Harry Truman. Dewey did not oppose Roosevelt's New Deal programs altogether, but merely campaigned on the promise that Republicans would run them more efficiently and less corruptly.
Let me warn the nation, against the smooth evasion which says, "of course we believe all these things, we believe in social-security, we believe in work for the unemployed, we believe in saving homes—cross our hearts and hope to die, we believe in all these things. But we do not like the way the president's administration is doing them. Just turn them over to us. We will do all of them—we will do more of them, we will do them better; and, most important of all, the doing of them will not cost anybody anything!
From 1936 to 1976, the more centrist members of the Republican Party frequently won the national nomination with candidates such as Alf Landon, Wendell Willkie, Thomas E. Dewey, Dwight D. Eisenhower, Richard Nixon and Gerald Ford. The mainstream of the Republican Party was generally supportive of the New Deal. In the 1950s, conservatives such as Robert A. Taft and Barry Goldwater, who rallied against "me-too Republicans", were considered outside of the mainstream of the then-centrist GOP; serious consideration was given to leaving the GOP and forming a new conservative party in coalition with the "states' rights" Democrats of the South.
In the 1960s and 1970s, Republicans considered liberal on domestic policy but hawkish on foreign policy were sometimes called "Nixonian", or "Rockefeller Republicans". While the term Nixonian took on other meanings after the Watergate scandal, neither expression had always been considered pejorative.
In the 1980s, the term gypsy moth Republican described Republicans from the Northeast and Midwest who voted against the Ronald Reagan administration's proposed cuts in aid to economically distressed people, contrasting with boll weevil Democrats, who voted for these cuts. The gypsy moth is an invasive species destructive to trees in the Northeastern United States.
In 2015 the term cuckservative, a portmanteau of cuckold and conservative, was popularized on the online forum 4chan, and embraced by both internet trolls and the nativist alt-right. The metaphorical " cuck" is represented in a genre of interracial pornography as a masochistic white husband who allows his wife to have sex with a stronger black man, thereby participating in his own symbolic emasculation. In white supremacist vernacular the term is an accusation of yielding to non-white interests on issues such as immigration or modern display of the Confederate flag; however, the term gained use (with some controversy) by more mainstream conservatives to denounce Republicans whose compromises included vote trading, rhetorical restraint in deference to donors, cooperation with Democrats on any particular initiative, or attempting to court voters by making appeals to supposedly liberal ideals.
Button of the week: It's slowly making the rounds as circulation is small but the "RINO" (pronounced "Rhino") could become a collector's item. Pictured is naturally the animal with a Ghostbuster's slash through it.
In the 1936 election, FDR had a field day with so-called "me too" Republicans.
... the recipe for a new conservative party was plain: one part Midwestern Taft Republican, one part Southern states' rights Democrat.
What was needed was a Northern counterweight to the "Boll Weevil Democrats," some 50 Southerners who consistently voted with [President Reagan] to whack at [aid to economically distressed people] ... some 20 Frostbelt Republicans have decided to defect from their lockstop White House support ...
... spread fast across the radical right. And ... found its way into the political mainstream. ... White supremacists ... tailor its definition to further describe politicians who don't fall in line with the white nationalist cause.
The radical nature of those ideas along with the pornographic connotations associated with "cuckold" have made the word a subject of hand-wringing among some conservative commentators.
The insult's most general gist is conservatives accused of bowing to one non-conservative idea or another, eg immigration reform, should feel humiliated, their ideology adulterated.