By "certain universities" we mean specifically Oxford (Cambridge apparently didn't require subfusc?), and can be approximately described as Edwardian white-tie wear. There's also an alternate adjectival form, subfuscous, for being like the clothing. As far as the clothing is concerned, the word was originally humorous slang first recorded in the 1850s, the general adjective having been adopted into English around 1710 from Latin subfuscus, dusky, from sub- in a little-known sense of approximately + fuscus, dark.
Thick carpet, subfusc curtains with a pseudo-empire pattern, and gilt-legged chairs give the waiting room the atmosphere of a mausoleum.