The young man's drug-addled mind made him speak incoherently.

drug-addled 是由 drug（毒品）和 addled 组成的复合形容词。addled 是动词 addle 的过去分词形式，原指"使（蛋）变质"，引申为"使头脑混乱"。drug-addled 形容"因吸毒而导致思维混乱的"。