Name comes from the Harry Potter character Dolores Umbridge. In Harry Potter and the Order of the Phoenix, Umbridge teaches at Hogwarts and is also in charge of all discipline. She frequently tortures students as punishment and is the most infuriating character in the series. HATED by all, she is rather passive-agressive, hiding her evil tendencies behind a disgustingly sweet and girly mask. For example, her office, where she forces Harry to carve the words "I must not tell lies" into his own skin, is entirely pink and covered with pictures of kittens. Even though she's technically not a "bad guy" because she doesn't work for Voldemort, she's easily the most hated character in the series.
OMFG I HATE *JANET* SO MUCH I CANT BELIEVE SHE SAID THAT TO ME THE FUCKING UMBRIDGE
Somebody obsessed with the Harry Potter series. Is sometimes used by non-Potter fans as an insult. More often used by the Pottergeeks themselves, and used with pride. A true Pottergeek can quote tons of scenes from the books and/or movies from memory, and notices right away if some random detail is off.
Pottergeek 1: WTF! They said Ginny's full name was Virginia!!!
PG 2: WHAT???? Everyone knows it's Ginevra!
PG 1: Haha, "Yes, my tiara sets off the whole thing nicely. But I must say, Ginevra's dress is far too low cut!"
PG 2: Seventh book, spoken by Auntie Muriel at Bill and Fleur's wedding scene!
PG 1: We are such Pottergeeks.
PG 2: Luv us!<3