The presence of borrowed or shared words in the Quran does not contradict its being "clear Arabic." A word can have an older foreign origin and still be fully Arabic in the speech, grammar, and understanding of the people to whom the Quran was revealed.
The strongest form of the question
The argument is not silly. Arabia was not sealed off from the world. Arabs traded with Syria, Iraq, Abyssinia, Yemen, and Persia. Arabic is a Semitic language, related to Hebrew, Aramaic, and others, so some words are shared by ancestry, while others were borrowed through contact. Critics point to words such as "Injil," "sijjil," "istabraq," or "qistas," and say: if the Quran calls itself Arabic, how can such words be in it?
They add that if Arabic itself developed through contact with other languages, then "pure Arabic" sounds like a myth, and the Quran looks like a human product of a mixed late antique environment.
What "clear Arabic" actually means
Indeed, We have sent it down as an Arabic Quran so that you may understand.
Quran 12:2
In a clear Arabic tongue.
Quran 26:195
The Quran does not claim to be written in a laboratory language with no historical contact with any other tongue. It says it is "Arabic" and "clear" - meaning its discourse, grammar, eloquence, address, and meanings were intelligible to the Arabs it first addressed.
This is how languages work. English contains words from French, Latin, Norse, Arabic, Hindi, and many others. Yet a sentence like "The judge signed the contract" is still English, though its vocabulary has mixed roots. Origin is one question; present linguistic identity is another.
Classical Muslim scholars discussed this carefully. Some, like al-Shafi'i, emphasized that whatever is in the Quran is Arabic because it was known in Arab speech. Others, such as al-Tabari and later scholars, allowed that some words may have foreign origins, but said they had been Arabized and absorbed into Arabic before revelation. These are not opposing faith positions so much as two ways of describing the same fact: the Quran speaks in the language of its first audience.
Loanwords do not make a text non-Arabic
A borrowed word becomes part of a language when its speakers use it naturally and shape it by their own pronunciation and grammar. The Quran's style is not Aramaic, Hebrew, Ethiopic, or Persian with Arabic decoration. Its syntax, morphology, rhetoric, rhyme, idiom, and force are Arabic. Its first hearers, including its fiercest enemies, did not say, "This is not Arabic." Their charge was different: they accused the Prophet (PBUH) of poetry, magic, or learning from others.
The Quran itself answers that kind of accusation:
And We certainly know that they say, "It is only a human being who teaches him." The tongue of the one they refer to is foreign, while this is a clear Arabic tongue.
Quran 16:103
Notice the point. The verse does not deny that Arabs knew words of outside origin. It denies that the Quran's source and composition can be explained by a non-Arab teacher, because the Quran itself is a sustained, masterful Arabic revelation.
What the argument proves - and what it does not
It proves that Arabic, like every living language, existed in history and interacted with neighboring cultures. It may also prove that some Quranic words have cognates or earlier roots in other Semitic or regional languages. That is not embarrassing; it is expected.
It does not prove that the Quran is confused about its own language. It does not prove that "clear Arabic" means "no borrowed vocabulary whatsoever." It does not prove human authorship. And the claim that even the word "Arab" must be Aramaic and therefore Arabic identity is derivative is, at best, an etymological debate, not an argument against revelation.
The Quran came down in the real Arabic of real people, not in an artificial tongue cut off from history. Its clarity lies in its address, meaning, and unmatched Arabic expression. Loanwords, where they exist, show how languages live; they do not turn Arabic revelation into something other than Arabic.