What is Snape's middle name?
This is something that people have been asking about for a while, and yet J.K. Rowling has never given us the answer. It is a simple question, and requires a one-word answer. Even so, there are elaborate arguments as to why one would guess one middle name rather than another.
A lot of people think it's Tobias, and I can see their reasoning. As creative and unique most of the names in the Potterverse are (most of which we had never even heard of prior to reading the series), the middle names tend to be named after that character's mother, father, uncle, grandfather, or someone else their parents knew. Harry James Potter. Draco Lucius Malfoy. Tom Marvolo Riddle. Ronald Bilius Weasley. Albus Severus Potter. James Sirius Potter. Lily Luna Potter. Teddy Remus Lupin. And I'd guess that Remus John Lupin's father's name was John, and that Hermione Jean Granger's mom's name was Jean.
So it would only make sense that Severus would be named partly after his father, since J.K. Rowling made everyone else too lazy to come up with their own name. Severus's mom, Eileen, must have really loved Tobias to stay with him even though he was abusive. I mean, she could use magic. If she didn't love him, then why didn't she just Imperius him or something? Besides, they were probably a happy couple before Severus was born, so Tobias wouldn't have minded naming his son 'Severus Tobias Snape.' And that does sort of have a ring to it, doesn't it?
The only argument I can find supporting that Tobias wasn't his middle name is that 'Eileen wouldn't have named her son after the Muggle husband she hated. And if Tobias had a say in naming his son, he wouldn't have wanted the wizard son he hated to have his name.' Well, I just proved that statement wrong in the previous paragraph. So that argument is invalid.
Basically, the only thing we have to go on his the possibility of him having his father's name for a middle name. Though it could very well be something entirely different, we have no evidence to support any names in particular unless J.K. Rolwing releases that information in Pottermore... I really hope she does. Or you could somehow try and talk to Severus and guess his middle name until he tells you.
Maybe it's 'Sexy'. Severus Sexy Snape... I like that.
It would be even better if he was Severus Freakin Snape.
OR if Eileen decided that his middle name should be 'Spy', so in the future when Voldemort and Dumbledore told him to do some spying and asked if he'd have any trouble, he'd be able to say, "Pfft. 'Spy' is my middle name... No really, it is. Take a look at my birth certificate." And of course you could also say the same about it being 'Dangerous' or 'Brave.'
