On first greeting a Sultan one would address him as Your Royal Highnesss/Your Majesty and subsequently as Tuanku. Tuanku is glossed in this context as Highness/Majesty. It literally means my lord - Tuan (lord) Aku/ku (my).
Until recently, Tuanku was also used as a title by older Sayyids (agnatic descendants of the Prophet Mohammed's grandson, Hussein) in Sarawak. This usage has been largely abandoned now under pressure from the sultans of west Malaysia, who seek to monopolise its use.