What is the longest one word palindrome and what does that word mean?

Question

you need to answer both to get this

Answer 1

You didn't say it had to be English. Since that's the case, it's saippuakuppinippukauppias which is Finnish for 'soap cup dealer.'

Answer 2

The longest non-English one word palindrome is:
saippuakuppinippukauppias (Finnish meaning 'soap cup dealer)- 25 letters
The longest English one word palindrome is :
tattarrattat (meaning rat-a-tat - see Oxford Dictionary) - 12 letters

Answer 3

English: 12 letters: TATTARRATTAT, a knock at the door, coined by James Joyce in "Ulysses". Somewhat suspect.
11 letters: DETARTRATED, contrived chemical term.
9 letters: ROTAVATOR (trade name), MALAYALAM (the language), REDIVIDER (not recognised in most dictionaries).
7 letters: RACECAR, longest ordinary word.

Finnish: 15 letters: SAIPPUAKAUPPIAS, a soap-seller. Said to be in common use. Longer related words can be contrived, such as the one quoted by gregory_dittman above, but are of more doubtful status.