Usul was a Fremen word meaning "the strength of the base of the pillar".
It was the name given to Paul Atreides when he was given sanctuary with Sietch Tabr by its naib, Stilgar. Openly Paul was to be referred to as Muad'Dib. However, Usul was the private name used within the Sietch. It was to be the name his beloved Chani would use for him, and the last word she cried before she died giving birth to their children.
Stilgar gave Paul this name after witnessing him defeat one of their finest warriors, Jamis. It would appear that Stilgar saw a certain strength in the boy.
Like various other Fremen words, the term "Usul" comes from Arabic, meaning "fundamental principle(s)".