So what does it mean? The most obvious explanation is a biblical reference is to Ismail, or Ishmael, son of Abraham. Muslim tradition says Ishmael rather than Isaac was the son Abraham was commanded to sacrifice. Ishmael's decedents went on to become Arab people, while Isaac's became the Hebrews.
This of course has sparked speculation that Cho was a religious extremist, fueled by the fact he left a note with a list of grievances including against "rich kids", "debauchery" and "deceitful charlatans".
Other explanations include literary references to Moby Dick or the James Fenimore Cooper novel The Prairie, and even confusing conspiracy theories about a link to an abduction of a South Korean boy in 1971.
Cho may have taken the meaning of Ismail Ax to his grave, but the phrase will live on. A TV repair shop owner in Corpus Christi, Texas, was quick to cash in on its notoriety. He registered the domain www.ismailax.com five minutes after he heard the phrase mentioned on a Fox News broadcast. Currently the site is a hotch potch of mixed messages, with ads for everything from car parts and sports results to university degrees.
Those searching for answers will just have to keep looking.