At first I thought he was being rude, but I've reached the conclusion that he's just shy.

conclusion 是名词，来自动词 conclude（得出结论）。con- 是前缀表示"完全"，-clude 来自拉丁语 claudere 表示"关闭"。合起来表示"完全关闭一个讨论或思考过程"，即"得出结论"。 在句中指经过思考后形成的最终判断或决定。