Watermelons are more expensive in winter because they're out-of-season.

out-of-season 是由三个部分组成的复合形容词。out 表示"不在...状态"，of 是介词"的"，season 指"季节"。组合起来字面意思是"不在季节内的"，引申为"反季节的"，指不在该水果自然成熟季节上市的农产品。