The most famous spy name of the 20th century is an ordinary Malay word for the sun. Mata means "eye" and hari means "day," so mata hari is "the eye of the day."
The woman who took it was not Malay. She was born Margaretha Gertruida Zelle in 1876 and grew up in the northern Netherlands. At 18 she answered an advertisement placed by Rudolf MacLeod, who wanted "a girl of pleasant character." They were engaged six days after meeting and married in July 1895. The couple spent years in the Dutch East Indies, and they separated in 1902.
In the Indies she had watched local dances, and National Geographic links her later name to them. Back in Europe and short of money, she reinvented herself. On 13 March 1905 she danced at the Musée Guimet, a museum of Asian art in Paris. Invitations went out to 600 of the city's wealthy elite. She billed her act as sacred temple dances from the Indies, a framing that helped her avoid charges of indecency.
The name did a lot of the work. It was a real phrase from a language she had heard in the colony, and it backed up the story of temple dances.
During the First World War she fell under suspicion. On 12 February 1917 the French issued a warrant for her arrest as a German spy, and she was arrested the next morning. She was executed by firing squad early on 15 October 1917.
The case against her has aged badly. National Geographic reports that the telegrams used as evidence are now considered to have been doctored, and the originals later disappeared. The prosecutor himself later said there was not enough evidence "to flog a cat."
Her stage name outlived her as a common noun. Wordsmith.org gives its earliest documented use for "a seductive woman who works as a spy" as 1936.
She was following an older pattern. In the 1840s an Irish woman, Eliza Gilbert, had found fame on stage as "Lola Montez, the Spanish dancer." Both women chose a foreign name to sell a foreign fantasy.