'Linen' in the Bible

Then Samson said to them, “Let me now ask you a riddle; if you can tell me what it is within the seven days of the feast, and solve it, then I will give you thirty linen tunics (undergarments) and thirty changes of [outer] clothing.

But if you are unable to tell me [the answer], then you shall give me thirty linen tunics (undergarments) and thirty changes of [outer] clothing.” And they said to him, “Ask your riddle, so that we may hear it.”