It is a "rule" created by grammarians that has no foundation. H.W. Fowler, writing 80 years ago, and Theodore Bernstein, writing 40 years ago, made plain that opposition to the split infinitive is a mere shibboleth.To me, a shibboleth is a pronunciation or other linguistic peculiarity that marks its user as belonging to a given group. It refers to the episode in Judges xii:5-6.
5 And the Gileadites took the passages of Jordan before the Ephraimites: and it was so, that when those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, Art thou an Ephraimite? If he said, Nay;
6 Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right. Then they took him, and slew him at the passages of Jordan: and there fell at that time of the Ephraimites forty and two thousand.
The American Heritage Dictionary provides this definition:
1. A word or pronunciation that distinguishes people of one group or class from those of another. 2a. A word or phrase identified with a particular group or cause; a catchword. b. A commonplace saying or idea. 3. A custom or practice that betrays one as an outsider.But clearly this is not the sense that McIntyre has in mind. Of the AHD definitions, only 2b might fit in the situation, but saying that something is merely a commonplace idea is not an attack on its veracity.
Merriam Webster gets closer.
1 a : a word or saying used by adherents of a party, sect, or belief and usually regarded by others as empty of real meaning1a is almost what we're looking for, but "opposition to the split infinitive" is nether a word nor saying. Interestingly, the three learner dictionaries that I consulted all provide definitions that match McIntyre's use exactly:
shibboleths come rolling off their lips -- Joseph Epstein> b : a widely held belief shibboleth is a myth -- L. A. Wood> c : TRUISM, PLATITUDE shibboleth that crime does not pay -- Lee Rogow>
2 a : a use of language regarded as distinctive of a particular group
shibboleth of social class -- Vivian Ducat> b : a custom or usage regarded as distinguishing one group from others shibboleth, its hour dividing mankind -- Osbert Sitwell>
- an old idea, custom, or principle that you think is no longer important or suitable for modern times -Longman Dictionary of Contemporary English (this is the only sense they define)
- 1 an old idea, principle or phrase that is no longer accepted by many people as important or appropriate to modern life: the crumbling of old political shibboleths -Oxford Advanced Learner's Dictionary
- If you describe a popular idea or belief as a shibboleth, you mean that it may be meaningless or wrong although many people believe it -Collins COBUILD English Dictionary.
The Online Etymology Dictionary explains, "Figurative sense of 'watchword' is first recorded 1638, and it evolved by 1862 to 'outmoded slogan still adhered to.'" The interesting thing about McIntyre's use is that it may help us see how shibboleth got from there to here. Use of the split infinitive by one group is used as a shibboleth by another (we have progressed from slaughter to mere disparagement.) However, a third group looks at this test and points out that the shibboleth has no factual basis.
It makes a good folk etymology at least.