The Quran

وَما كُنتَ تَتلو مِن قَبلِهِ مِن كِتابٍ وَلا تَخُطُّهُ بِيَمينِكَ ۖ إِذًا لَارتابَ المُبطِلونَ

Muhammad Asad

for, [O Muhammad,] thou hast never been able to recite any divine writ ere this one [was revealed], nor didst thou ever transcribe one with thine own hand¹ - or else, they who try to disprove the truth [of thy revelation]² might indeed have had cause to doubt [it].

Lit., "with thy right hand" - the term yamin being used here metonymically, denoting no more than one's "own hand". - It is historically established that Muhammad, the "unlettered prophet" (cf. 7:157 and 158), could neither read nor write, and could not, therefore, have derived his extensive knowledge of the contents of earlier revelations from the Bible or other scriptures: which - as the Qur'an points out - ought to convince any unprejudiced person that this knowledge must have come to him through divine revelation.
The participial noun mubtil is derived from the verb abt ala, "he made a false [or "vain"] claim", or "tried to disprove the truth [of something]", or "to reduce [something] to nothing", or "to prove [it] to be of no account", or "null and 'void", or "unfounded", "false", "spurious", etc.; irrespective of whether the object is true or false, authentic or spurious, valid or unfounded (Lisan al-'Arab and Taj al-'Arus).