Mr. Moony presents his compliments to professor Snape, and begs him to keep his abnormally large nose out of other people's bussiness.
Snape froze. Harry stared, dumbstruck, at the message. But the map didn't stop there. More writing was appearing beneath the first.
Mr.Prongs agrees with Mr.Moony and would like to add that Professor Snape is an ugly git.
. . .
Snape pointed at the parchment, on which the words of Messrs. Moony, Wormtail, Padfoot, and Prongs were still shining. An odd, closed expression appeared on Lupin's face.
Harry potter and the Prisoner of Azkaban
Here, Snape found out that Harry was possessing this magical map, and called Lupin to ask him what this is. It turns out later in the book that Lupin is, in fact, Mr.Moony, which means he was one of the four who made the map which teases Snape. But Harry knows none of these yet.
In this situation, what does "closed" expression mean? I thought from the context that Lupin seems to or wants to hide something but not sure what exactly it means.
Can anyone help me with this expression