Lisan al Gaib is the Fremen term for an off-world prophet.
It is "The Voice from the Outer World" and is outlined in Fremen messianic legends heavily influenced by the Bene Gesserit's Missionaria Protectiva. It is also translated as the "Giver of Water."
The term comes from the Arabic, "لسان," (Lisan) meaning "Tongue," and "الغيب" (al Gaib) meaning "The hidden" or "unseen". It should be noted that in context of Islam, "الغيب" (al Gaib) signifies the 'unseen dimensions of Reality', and the "tongue of the unseen" is a "prophet", and not a "foreigner".
Beginning in The Original Dune, Paul Atreides is often referred to as the Lisan al Gaib by the Fremen. This is first seen when the Atreides arrive on Dune and are ‘touring’ the capital.