Marabout is a French rendering of the Arabic murabit, a person stationed in a ribat. A ribat was originally a fortified post on a frontier, and its garrison combined military duty with devotional discipline, so the word carried both senses from early on. As the institution moved inland across the Maghreb and the Sahara, the fortification fell away and what remained was the association of a place, a lineage and a reputation for holiness. By the time European observers were writing about it, a marabout might be the head of a Sufi lineage, the keeper of a Quran school, a maker of written amulets, a mediator in disputes between families, or a man whose principal qualification was descent from someone who had been all of these.
That range is the first thing to hold on to about the word, because it is not one role. The second is the relationship to lineage. Holiness in this system is inherited as well as achieved: baraka, the blessing or effective power attributed to a holy person, is understood to persist in his descendants and in his tomb, so a family can hold religious standing across generations without any individual member having done anything remarkable. Whole social structures rest on this, with maraboutic families occupying a distinct position beside warrior and merchant groups.
The word is also, unavoidably, a colonial artefact. French administrators used it for almost any Muslim religious figure they had to deal with, and the ethnography written in that period carried the flattening into the scholarly literature, where it is still being undone.