From Everything You Know about English is Wrong:
Now that you know, it's time to, well, bite the mother tongue. William Brohaugh, former editor of Writer's Digest, will be your tour guide on this delightful journey through the English language, pointing out all the misconceptions about our wonderful – and wonderfully confusing – native tongue. Tackling words, letters, grammar and rules, no sacred cow remains untipped as Brohaugh reveals such fascinating and irreverent shockers.
What does the phrase/idiom "no sacred cow remains untipped" mean?
Best Answer
Not an idiom, but a mash-up of two expressions.
A sacred cow (from the Hindu veneration of cattle) is an idea or principle that is held to be above criticism, especially one which the author believes is unreasonably so.
A sacred cow in English grammar might be "Every sentence has a verb". Really!
Cow tipping is the (urban legend) of a pastime in which people sneak up beside cows and push them over.
These two ideas are combined for comic effect. The pattern is similar to "no stone is left unturned" (meaning every place has been searched). Altogether it means that every principle of Engish grammar is challenged.