Longest word in Arabic made me giggle.
The longest word in Arabic mentioned in The Holy Qur'an is -فأسقيناكموه- or -فَأَسْقَيْنَاكُمُوهُ-, which is 11 letters, and means "and we give it to you to drink
PNEUMONO*ULTRA*MICRO*SCOPIC*SILICO*VOLCANO*CONIOSI S (also spelled PNEUMONO*ULTRA*MICRO*SCOPIC*SILICO*VOLCANO*KONIOSI S) = a lung disease caused by breathing in particles of siliceous volcanic dust.
This is the longest word in any English dictionary. However, it was coined by Everett Smith, the President of The National Puzzlers' League, in 1935 purely for the purpose of inventing a new "longest word". The Oxford English Dictionary described the word as factitious. Nevertheless it also appears in the Webster's, Random House, and Chambers dictionaries.