"If [Snape] sees you .... How do you open the witch's hump again?"
-- Hermione, volunteering to recover the invisibility cloak Harry had left under the statue (PA16)
Halfway along a third-floor corridor between the staircase leading from the Entrance Hall and the way to Gryffindor Tower, stands a statue of Gunhilda of Gorsemoor, referred to by the students as “the one-eyed, hump-backed witch.” This statue conceals the entrance to a secret passage leading to Honeydukes’ cellar (PA10). One opens the secret passage by tapping the statue and saying “Dissendium” (PA10, PA16).
Gunhilda of Gorsemoor was a healer, who lived from 1556 to 1639. She was a one-eyed, hump-backed witch famous for developing a cure for Dragon Pox. When she died, everyone who knew her felt great sorrow, for she was a talented and hardworking healer. She is featured on a Famous Wizards/Chocolate Frogs Card.
The name "Gunhilda" is Nordic and derives from the elements gunnr, "war," and hildr, "battle."
The location of the statue is revealed in these quotes:
He turned, halfway along the third-floor corridor, to see Fred and George peering out at him from behind a statue of a humpbacked, one-eyed witch (PA10).
Harry couldn't help noticing that the statue of the one-eyed witch on the third floor remained unguarded and unblocked. It seemed that Fred and George had been right in thinking that they -- and now Harry, Ron, and Hermione -- were the only ones who knew about the hidden passageway within it (PA14).