What is meant by "tongue" (لِسَان) in the verse: "Those who disbelieved among the Children of Israel were cursed by the tongue of David and Jesus, son of Mary. That was because they disobeyed and were [habitually] transgressing," and why was the dual form "tongues" (لِسَانَي) not used?

The meaning of cursing the disbelievers through the tongue of David and Jesus, peace be upon them, is to supplicate against them verbally. Al-Tahir ibn Ashour stated that the curse was through their words, which were constantly on their tongues. Some books of exegesis mention that David supplicated against the people of Aylah when they transgressed on the Sabbath, saying: "O Allah, clothe them with the curse like a cloak, and like a girdle upon the loins," whereupon Allah transformed them into apes. And Jesus supplicated against the companions of the Table when they disbelieved, saying: "O Allah, punish whoever disbelieved after eating from the Table with a punishment You have not inflicted upon anyone else in the worlds, and curse them as You cursed the companions of the Sabbath," whereupon they became pigs.

As for the reason for using the singular form "tongue" (لسان) instead of the dual "two tongues" (لسانَي), linguists have clarified that when a dual noun is attributed to two things that are part of it, it is permissible to use the plural, the dual, or the singular. The most eloquent form is the singular when the two things are distinguished, as in the Almighty's saying: "through the tongue of David and Jesus, son of Mary."