A udant is a familiar term for a friend or someone of relative proximity. It is used in frustration and in a strained tone. The frustration though is not genuine, it is something that the person doesnt actually really mind about but is pretending that they do, hence the strained voice.
Tom is in the library trying to work. Walter is next to him and putting him off his work, wasting time on sites like the urban dictionary. Tom is clearly not minding being put off as it means he doesnt have to work, but might say the term udant in a strained tone to express frustration, but not actually.