Allah tells us about the idolators' lies, allegations, and slander when they claimed that this Qur'an which Muhammad ﷺ had recited for them, was actually taught to him by a human. They referred to a foreign (i.e., non-Arab) man who lived among them as the servant of some of the clans of Quraysh and who used to sell goods by As-Safa. Maybe the Messenger of Allah ﷺ used to sit with him sometimes and talk to him a little, but he was a foreigner who did not know much Arabic, only enough simple phrases to answer questions when he had to. So in refutation of their claims of fabrication, Allah said:

(The tongue of the man they refer to is foreign, while this (the Qur'an) is a (in) clear Arabic tongue.) meaning, how could it be that this Qur'an with its eloquent style and perfect meanings, which is more perfect than any Book revealed to any previously sent Prophet, have been learnt from a foreigner who hardly speaks the language? No one with the slightest amount of common sense would say such a thing.