75:17 It is up to Us to gather and preserve it and the reading of it. [He will bless whatever effort we put into its learning]
75:18 Thus, when We read it, follow the reading. [Although these verses are addressed to the exalted Prophet, every believer can sense as if the Qur’an is talking to him/her]
75:19 Then, it is for Us to explain it. [The Qur’an is its own best commentary. It explains itself]
Dear brother Kian,
Even today, the top source of understanding the Quraish dialect is the largely surviving pre-Islamic JAHILIYYAH poetry.
Secondly, Bedouins living in the Empty Quarter (Eastern province of the Arabian Peninsula) even now speak the Quraish dialect of Arabic. [I have lived among them, and many scholars did in the 19th and 20th centuries.]
Dictionaries come much later.