In Chapter 1, we meet various of the characters who are headed for Indian Island. We get to know how they have been lured to the island. In the case of Judge Wargrave, he believes that he has been invited by a friend of his, Constance Culmington. She is apparently the sort of person who is prone to doing things on whims and someone who gets somewhat strange enthusiasms. It is in this context that we see the words that you are asking about.
Contadini is a plural word. It is the plural of contadino, which means “peasant” or “farmer.” So, when Culmington went to Italy, she wanted to really immerse herself in the culture and become like a peasant (or so she said). Similarly, when she went to Syria, she wanted to become like the Bedouin. Bedouin is the term that is used to refer to Arab nomads in the Middle East.
This passion that she has for throwing herself into roles like this explains why Wargrave is not surprised to think that she has bought an island and started to act eccentrically.