The mother of all means the ultimate, the biggest of something, of anything, the standard by which all others are gauged. Frequently, it is used with a tone of irony or mockery.

The phrase was popularized in the American language by Iraqi dictator Saddam Hussein. He used it in a 25-minute speech to his nation on January 6, 1991, on the eve of what would become the first Gulf War between Iraq and the U.S.-led coalition. That war forced Iraq to retreat from Kuwait, which Saddam's forces had invaded the previous August.

Saddam told his army that it was fighting against social and economic oppression, against discrimination, and against the oppression of foreign power, "against double standards, corruption, and hegemony. For these reasons," he said, "the battle in which you are locked today is the mother of all battles." In his use of that term he was hearkening back to the original mother of all battles in Arabic history (umm al-ma'arik), the battle of Qadisiya in 637, in which a united Islamic force defeated Persia.

The mother of all caught on; citations show plenty of evidence both in the last decade (and in the last century) and in this one.

Here is the mother of all in an article by Jon Anderson in the Chicago Tribune, dated December 31, 1996:

"Fact is, in these waning years of the 20th Century, all remaining New Year's Eves are but pale warm-ups for what is coming to a time zone near you. Call it the Night of the Millennium. Or simply the big one.
Three years before the onslaught of the Mother of All New Year's Eves, a startling 183 rooms have been booked at the Drake Hotel for the historic night of the big change."

And, in July 2005, editors at Scientific American gave an article on stem cells this headline: Mother of All Cells.

In the first few years of the second millennium, author Ann Douglas published The Mother of All Pregnancy Books, The Mother of All Baby Books, The Mother of All Toddler Books, and The Mother of All Parenting Books.