Here's how:
* The word in question is "lustrous." We don't know what it means.
* The sentence describes the material as "lustrous messaline" and then states it is being used to make a "beautiful gown." This implies that "lustrous" is a positive quality that contributes to the beauty of the gown.
* Therefore, we can infer that "lustrous" means something like shiny, sparkly, or having a beautiful sheen.
By relating the unknown word ("lustrous") to the known qualities of the material and the desired outcome (a beautiful gown), the sentence effectively defines the word for the reader.