The phrase appears variously as deus vult, dieu le veut, deus lo volt, etc etc.and even though every book about the crusades mentions it in one form or another, no one ever says where it comes from.
'Lo' is Italian for 'it.' 'Volt' is third person singular for the Medieval French verb Voler - to will, wish, desire, want. This article originally said It means 'God wills it' in Lingua Franca, an international language (similar to some degree to the modern Interlingua) of Western Europe. There is already some discussion about this on Talk:First Crusade.
