What is the longest word in Qur'an and in which surah?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The longest word in the Quran is: (فأسقيناكموه) "Fa asqaynaakumooh" [11 letters] Surah 15: verse 22. The meaning of the word means {…and given you drink from it.}.

The whole verse is which the above word is mentioned is as follows:

Allah Says (what means): {And We have sent the fertilizing winds and sent down water from the sky and given you drink from it. And you are not its retainers.} [Quran 15:22]

Allah knows best.