He was so engrossed in what he was doing that he didn't hear the bell.

engrossed 是动词 engross 的过去分词形式，表示"全神贯注的、专心致志的"。engross 由前缀 en-（表示"使进入某种状态"）和 gross（古法语词根，意为"大的、全部的"）组成，字面意思是"使完全投入"，引申为"使全神贯注"。