-- George Weasley to the Fat Lady (GF12)
A painting of a large woman in a pink silk dress hangs over the round doorway into the Gryffindor Common Room. To gain admittance to through the portrait hole, a person must give the correct password to the Fat Lady, in which case she swings her frame out from the wall. She has been known to get a bit testy with people who wake her up for no good reason and even to wander off late at night out of her frame to go visiting, making it impossible to get into Gryffindor Tower. She is friends with Violet (GF17). At Christmas, The Fat Lady and Violet tend to consume a good bit of holiday cheer; during Harry’s fourth year they got a bit tipsy on chocolate liqueurs (GF23), and over the holidays of Harry’s sixth year they drank their way through quite a lot of five-hundred-year-old wine (HBP17).
In Harry’s third year, Sirius Black broke into Hogwarts and viciously attacked the Fat Lady’s portrait, ripping the canvas to shreds, making her flee her frame (PA8). She was found “hiding in a Map of Argyllshire on the second floor” (PA9). While she recovered, a painting of Sir Cadogan and his gray pony guarded the Gryffindor Common Room. When the Fat Lady was restored, she resumed her place but asked for added protection by security trolls (PA14).
When Dumbledore died, she allowed Harry into the common room without requiring a password, but wailed when he told her it was true (HBP29).
Passwords used to enter Gryffindor Tower through the Fat Lady's portrait:
caput draconis (PS7)
Latin for "dragon's head".
pig snout (PS9)
Latin for "better luck" (PA5)
scurvy cur (Sir Cadogan thought this one up) (PA11)
oddsbodikins (Sir Cadogan thought this one up too) (PA12)
The most common modern meaning of this word is "an irresponsible, silly person", but a much rarer meaning implies that the person is a mischief-maker if not actually a fiend. Think Peeves for the rarer sense of the word. Perhaps refers to Neville Longbottom who kept leaving passwords lying about.
fairy lights (GF22)
banana fritters (GF25)
Mimbulus mimbletonia (OP11)
A kind of stew or porridge that historically was offered to the British monarch on coronation day by the lord of the manor of Addington, Surrey (NSOED)
Post-Christmas holidays password, after the Fat Lady and her friend Violet drank all the wine in a picture of a group of drunk monks in the Charms corridor (HBP17)
quid agis (HBP24)
Latin for "How do you do?"
The name probably refers to the well-known phrase "It ain't over till (or until) the fat lady sings," which suggests that one shouldn't assume the outcome of a situation until it's actually over. The phrase is commonly used in sporting events, when the winner shouldn't be assumed since anything can happen. According to Wikipedia,
The phrase is generally understood to be referencing the stereotypically overweight sopranos of the opera. The imagery of Richard Wagner's opera cycle Der Ring des Nibelungen and its last part, Götterdämmerung, is typically the one used in depictions accompanying reference to the phrase. The "fat lady" is the valkyrie Brünnhilde, who is traditionally presented as a very buxom lady with horned helmet, spear and round shield (although Amalie Materna played Brünnhilde during Wagner's lifetime (1876) with a winged helmet). Her aria lasts almost twenty minutes and leads directly to the end of the whole Ring Cycle. As Götterdämmerung is about the end of the world (or at least the world of the Norse gods), in a very significant way "it is [all] over when the fat lady sings."
While there is nothing in canon to identify the Fat Lady with an opera singer, the film version of Prisoner of Azkaban does play on this association when the Fat Lady tries to break a wineglass with her singing voice (PA/f).