He is supposedly an expert in this field, but his answers were vague.

supposedly 是由动词 suppose（假设、认为）加上副词后缀 -edly 构成的副词。suppose 的意思是"假设、认为"，加上 -edly 后表示"据推测、据说"，常用来表示某种说法或观点并非完全确定或可能存疑。