In his book Ma'na al-Nakba (The Meaning of the Disaster), Constantin Zureiq described the Arab defeat not just as a military loss, but as a civilizational setback. He believed the Arab states had been unprepared, disorganized, and overconfident, and that their failure to prevent the establishment of Israel revealed deeper problems in Arab society—like a lack of modern institutions, unity, and strategic thinking.

i.e the failure to prevent Israel existing.

From the River to the sea Palestine will be Arab indeed

Answer: a war of genocide and extermination attempted by 6+ Arab/muslim majority states against the nascent nation of Israel, fortunately for all involved they completely failed and in fact ended up losing land. While they did succeed in ethnically cleansing the Jews from any land that did end up under Jordan and Egyptian occupation, and they did expel (ie ethnically cleanse) Jews from most Muslim majority countries, they failed in their overarching goal of finishing Hitler’s work of exterminating all Jews.

Certain morally degenerate groups see this failure (of killing all Jews) as an absolute catastrophe and thus name it the Nakhba.