Turns out to be an insult hurled at infidels.
What You Heard About "Allahu Akbar " Is WRONG. Here's What It Really Means
Greenfield adds that the true message of "Allahu akbar" is outlined in the Quran 61:9, which reads, "He [Allah] it is who has sent his messenger [Muhammad] with guidance and the religion of truth to make it victorious over all religions, even though the infidels may resist."
Resistance of the infidels, or refusal to convert to Islam, is the reason for Allah's command to behead, enslave, and conquer. Until the Caliphate is established, implementing Sharia law in every nation on earth, Muslims must not rest, but are required to continue violent jihad.
Until the left will acknowledge that Islam is not a religion, but is a political ideology hellbent on conquering the world, terrorist groups like ISIS will continue to slaughter and expand their tyranny, resulting in the defeat of the West by our own political correctness.