Fedaykin was originally a word used to describe the Fremen's guerrilla fighters, and was later used in reference to Muad'Dib's personal guard—his death commandos. By the time Leto and Ghanima Atreides were born, the term Fedaykin had become synonymous with the elite of Muad'dib's Jihad army.
However, after Paul was blinded and vanished into the Arrakis desert, the Fedaykin's fortunes faltered. In 10,210 A.G. Alia Atreides dissolved the Fedaykin, and the term reverted to its original Fremen meaning.
The word "Fedaykin" is derived from the Arabic word Fedayeen which denotes a member of an Arab commando group.