The short and uncomfortable truth is that the phrase is a standard expression of faith for roughly every Muslim on earth, and violent extremists have borrowed it for the same reason that political movements borrow flags: it lends their actions an air of legitimacy. Understanding that gap between everyday meaning and criminal misuse explains the whole puzzle.
What the phrase means
"Allahu akbar" is Arabic and translates as "God is greater", and it is often rendered as "God is the greatest". Muslims say it in prayer and at the call to prayer, and also in daily life to express admiration, gratitude, surprise or grief. A teaching professor of Arabic at Northeastern University has described people using it to admire a nice car or beautiful music, which shows how ordinary the expression is.
Why extremists use it
Violent groups try to present their crimes as religious duty, and a famous religious phrase helps them do so. Attackers have shouted it in several high-profile incidents, including the 2015 Charlie Hebdo attack and a 2013 attack on a British soldier, and the repeated media coverage has fixed the association in the minds of many listeners. The same professor says extremists have hijacked the phrase and wrongly tied it to scripture, noting that the form they use does not appear in the Quran in that way.
The words are not the weapon
A linguistics professor quoted in the same article argues that the words themselves are not the problem and that people misuse them. Her comparison is with the way Japanese and German were treated in the United States during wartime, when an entire language became suspicious because of an enemy. Violence has causes in ideology, grievance, recruitment and politics, and those causes would exist whatever slogan was chanted.
Why the distinction matters
Treating the phrase as a warning sign insults the vast number of Muslims who say it peacefully every day.
Assuming it explains an attack distracts from the real drivers of extremism, which investigators examine case by case.
Muslim scholars and organisations have repeatedly condemned the use of the phrase to justify killing, calling it a distortion of their faith.
How to think about it responsibly
When news reports say an attacker shouted the phrase, that detail describes a behaviour and not a motive proven by religion itself. Responsible coverage separates the criminal from the community and avoids implying that a whole faith stands behind an act that most of its followers abhor.
A clear answer
Terrorists shout the phrase because it is familiar, powerful and widely understood, and they hope it will give their violence a religious cloak. The ones who say it in prayer, in celebration or in sorrow far outnumber those who abuse it, and recognising that difference is the first step towards judging events accurately.