know-all
n (infml derog 口, 贬) person who behaves as if he knows everything 自以为无所不知的人
one of those young know-alls fresh from university 大学刚毕业的一个小小的万事通.
Though people have often laughed at stories told by seamen, it is now known that many of these 'monsters' which have at times been sighted are simply strange fish.