"Allahu Akbar" is an Arabic expression meaning 'Allah is greater' or 'Allah is the greatest'.
The reason for such an expression existed even BEFORE Muhammad and his Quran since Allah was the SUPREME rock god of Pagan Mecca among the other 360 rock gods and hence was AKBAR/ GREATER than ALL the others.
Sunan of Abu-DawoodHadith 774 Narrated byAbuSa'id al-Khudri
When the Apostle of Allah (peace be upon him) got up to pray at night (for tahajjud prayer) he uttered the takbir and then said:
"Glory be to Thee, O Allah," and "Praise be to Thee" and "Blessed is Thy name," and Exalted is Thy greatness." and "There is no god but Thee." He then said: "There is no god but Allah" three times; he then said:
"Allah is altogether great" three times: "I seek refuge in Allah, All-Hearing and All-Knowing from the accursed devil, from his evil suggestion (hamz), from his puffing up (nafkh), and from his spitting (nafth)" He then recited (the Qur'an).
"Allahu Akbar" became - and continues to be - the Battle Cry of Muhammadan Islam and is also called the TERROR verse.