What is the longest English palindromic word?

The longest English palindrome in the dictionary is TATTARRATTAT (12 letters), a word made up by James Joyce to indicate the sound of someone knocking on the door.

The word DETARTRATED (11 letters) is also considered by some to be a word. It is a chemical term. Another long one is KINNIKINNIK (11 letters), a type of tobacco substitute used by North American Indians.

I can offer two possible contenders, both of nine letters. They are: evitative and redivider

There is also a contrived chemical term, detartrated, which has 11 letters. I note that you specify English words.

The longest English palindromic word is redivider (9 letters).

The longest palindromic word in any language is saippuakivikauppias (19 letters), Finnish for lye dealer, from saippuakivi, lye, and kauppias, dealer. Similarly: saippuakauppias (15 letters), soap dealer, and saippurakaruppias (17 letters), soap salesman.

Kinnikinnick, the name of a plant and also a dried leaf and bark mixture smoked by Cree Indians, is not a palindrome, but it is sometimes misspelled as a palindrome in two different ways: kinnikinnik (11 letters) and kinnik-kinnik (12 letters). Each half of the last palindrome is itself a palindrome.

The shortest palindromic word, not counting one-letter words, is aa, which is a Hawaiian word for a kind of lava.

TATTARRATTAT is the longest palindrome in the OED2, which calls it a nonce word. The OED2 shows a single use in 1922 by James Joyce in 'Ulysses': "I knew his tattarrattat at the door."

https://www.proprofs.com/quiz-school/story.php?title=mtaymdy5ma==8s7q

The longest known palindrome – a word that reads the same backwards or forwards – in any language is the 19-letter word saippuakivikauppias

(According to Guinness Book of Records)

Saippuakauppias, Finnish for "soap vendor", is claimed to be the world's longest palindromic word in everyday use. A meaningful derivative from it is saippuakalasalakauppias (soapfish bootlegger). An even longer effort is saippuakuppinippukauppias (soapdish batch seller).

Koortsmeetsysteemstrook, is probably the longest palindrome in Dutch, and Kuulilennutunneliluuk (trajectory tunnel hatch) is the longest palindrome in Estonian.

https://en.wikipedia.org/wiki/Palindrome

A 17,259 word palindrome (or palindromic sentence)is found on this link:

http://www.norvig.com/pal2txt.html

Check this for a 63,647 letter palindrome:
a 15,139 Word Palindrome (or Palindromic Sentence)

http://www.norvig.com/pal1txt.html

Multi-Word Palindrome:

Jerry Berns has his candidate longest palindrome, consisting of 31,358 words or 119,180 letters.

https://drcrazie.blogspot.com/2016/02/

Single Word Palindrome

This link is quite interesting.

In English, it mentions tattarrattat as the longest, but Guinness says it is detartrated (the preterite and past participle of detartrate).

Rotavator is also a long palindromatic word.

Of course, using other languages, words like Kuulilennuteetunneliluuk, saippuakuppinippukauppias, and Koortsmeetsysteemstrook are considered the longest palindromatic words. Keeping in English, tattarrattat wins, followed by detartrated. Here is a link.

https://en.wikipedia.org/wiki/Palindrome#Long_palindromes

http://www.kmjn.org/notes/word_unit_palindromes.html
When the words in a sentence are taken as the units for making the palindrome, rather than the individual letters. An example is:
"Mind your own business: own your mind"