After months of unfulfilled promises, the company's excuses are beginning to wear thin with investors.

wear 在这里是动词，意思是"磨损、消耗"，引申为"逐渐减弱"。 thin 是形容词，意思是"薄的、稀薄的"，引申为"不充分的、站不住脚的"。 wear thin 是一个固定短语，字面意思是"磨损变薄"，比喻"逐渐失去说服力或可信度"。