The description is given in the episode "Graveyard Shift." He has a rusty spatula for a left hand, and has dark red eyes.
Squidward created the character of the "Hash-Slinging Slasher" in order to scare SpongeBob as a form of entertainment during the 24-hour night shift.
According to Squidward's story, the Hash-Slinging Slasher was a former fry cook who worked at the Krusty Krab before SpongeBob and was more clumsier than him. The Slasher one night accidentally severed one of his hands by mistake while cutting Krabby Patty meat, and replaced it with a rusty spatula. Later, when he was going home, he was fatally run over by a bus and "fired" at his funeral. Out of vengeance, his ghost returns to the restaurant every Tuesday night via the bus that killed him and "gets" unsuspecting victims. However, his presence is announced by (in sequential order) flickering lights, a phone call with nobody on the sending line, and the arrival of the bus that killed him, stopping just outside the Krusty Krab.
Squidward later confessed that he fabricated the story in response to SpongeBob's almost frenetic and melodramatic screams of terror. Later that night, however, it seemed that the events of his story were coming true. Eventually, it was revealed that the Hash-Slinging Slasher, or what they thought was him, actually came to apply for a job at the Krusty Krab. He was also revealed to be the cause of the latter two "signs" signalling the Hash Slinging Slasher's arrival, with Nosferatu switching the lights on and off.
The Hash-Slinging Slasher's name is often mispronounced by SpongeBob, and to a lesser extent, Squidward. This made one of the most memorable quotes from the episode.
Squidward: You mean you've never heard the story of the... hash-slinging slasher?
SpongeBob: The slash-bringing hasher?
Squidward: The hash-slinging slasher.
SpongeBob: The sash wringing... the trash thinging... mash flinging... the flash springing, bringing the the crash thinging the...
Squidward: Yes. The hash-slinging slasher.
In the Croatian version of the show, The Hash-Slinging Slasher's name is "Jack Konobarosjek", which is a parody of "Jack The Ripper" (Jack Trbosjek in Croatian).
The Hash Slinging Slasher is a parody of famous slasher film villains such as Freddy Krueger from "Nightmare on Elm Street" and Jason Voorhees from "Friday The 13th".
The Hash Slinging Slasher can also possibly inspired by the Hook-handed man, an urban legend about a serial killer who replaced one of his hands with a hook, much like the Hash-Slinging Slasher, although the Hash-Slinging Slasher replaced his hand with a rusty spatula instead of a sharp hook.
The Hash-Slinging Slasher appears in Squidward's stage (B-Movie) in SpongeBob SquigglePants, which was a prank by SpongeBob and Patrick to scare Squidward, if the player loses all of their lives, SpongeBob and Patrick reveal themselves and scare Squidward, but if the player wins, SpongeBob and Patrick fall, with Squidward looking in annoyance.
In the Bahasa Indonesia version of the episode, the cause of Hash-Slinging Slasher's death was different: instead of dying by a bus crash and being fired at his funeral, he was killed due to burning alive (which is implied to be due to spontantenous combustion) and shortly after goes crazy, after replacing his hand with a spatula.
To "sling hash" means to work at a restaurant, mainly a fast food one.