«عَلِّمْ فِي الْمِتْبَلِّمْ يِصْبَحْ نَاسِي»

The saying pictures someone pouring effort into teaching a dim-witted person through the evening, only to find that by daybreak every lesson has evaporated. The labor is real, but nothing sticks, because the learner lacks the capacity to hold on to what is given.

More broadly, it voices a traditional judgment that some people are simply unsuited to instruction, and that persistence alone cannot overcome a mind that will not retain. It casts the teacher's diligence as futile and places the failure squarely on the learner, a harsh verdict the saying asserts rather than one to be accepted as fair.

Said with exasperation about someone who cannot retain instruction despite patient effort, or to warn a would-be teacher against wasted energy. Applied to a real person it is a blunt insult, labeling them a fool, so it suits frank complaint among confidants rather than remarks to the person concerned.