Meezab is a golden spout from which rainwater on the roof of the Kaaba falls on the hijr Ismail. It is reported that the Prophet (saw) prayed at the place.
It is commonly referred to as "Meezab-e-Rahmah" (Mercy Drain).
The Quraysh were the first to build a roof on the Kaaba, and therefore the first to attach this drain pipe. Before their construction, there was no roof or drainpipe.
Meezab was reconstructed when the Kaaba was renovated in 1417 A.H. The dimensions were kept the same as they were when Ottoman Emperor Sultan Abdul Majeed Khan renewed Meezab in 1273 A.H.
Recommendations: Guide to visiting Mecca and Madina - Sh. Abdul Aziz Ahmed, The History of Mecca Mukarramah - Dr. Muhammad Ilyas Abdul Ghani