He seems quite knowledgeable about sex and always gives reasonable advice to his friends on such topics.

knowledgeable 是由名词 knowledge（知识）加上后缀 -able（能够...的）构成的形容词，表示"有知识的、博学的"。 about sex 是介词短语，about 表示"关于"，sex 指"性"。 knowledgeable about sex 整体意思是"在性知识方面有见识的、对性话题有经验的"。