«يِبِيعِ الْمَيَّهْ فِي حَارْةِ السَّقَّايِينْ»

The saying pictures a vendor trying to sell water in the very neighborhood where water-carriers live and work; the word for the place can mean a lane, but here it means the whole district. Nobody there needs what he sells, so his effort is wasted. The image points to an offer badly misplaced among people who already have plenty.

More broadly, it criticizes putting something where it does not belong or has no value. A speaker may use it to tease someone who shows off a skill in front of masters of that skill. It can also describe a plan that ignores the obvious realities of its setting.

Use it about someone who offers goods, advice, or skills to people who already have plenty of them, such as lecturing experts on their own field or bringing a gift the host already has in abundance. The tone is wry or mocking.