Ha Ha! is Nelson's catchphrase and iconic laugh. Nelson says the laugh with the emphasis on the first "Ha," so it sounds like "Ha-ha!".
Nelson usually uses the laugh when something bad happens to another character. Sometimes, after he uses the laugh, he says why he is laughing at the person. For example, he says, "Ha ha! Your dad's not handy!" to Bart after his ten-speed bike (which Homer had assembled) fell apart while Bart was riding it.
When Nelson laughs at Martin shortly after punching him in the gut when the latter's birthday party was forced to end early due to the guests falling ill from food poisoning from oysters served by Mrs. Prince, Nelson proceeds to do his trademark laugh, only for his second "ha" to be drawn out into more of a pained groan as he himself succumbs to food poisoning.In “Bart the General”, Nelson's first appearance, he laughs with three "Ha"s, particularly when Bart imagines Nelson as a giant who eventually eats him up, and in “The War of the Simpsons” he laughs like this to Abe Simpson
When Nelson laughs at Homer while he (Homer) is having a chili-pepper-induced hallucination, Homer hears the laugh with the emphasis reversed: "Haw-Haw!".
Sometimes, Nelson will start the laugh, and then partway through realize that the joke is actually on him, and the laugh comes out sounding like a question: "Ha-Ha?". In TOHXIV, He said "Ha-huh?".
The Irish Nelson says "Har! Har!".
When Bart jumps on the see-saw and the rubbish bag on the other side lands on him, Nelson says "See-Saw" in the same sort of way as "Ha Ha!.
Nelson trips up Facebook founder Mark Zuckerberg in a roller-skating rink. Zuckerberg falls and hurts himself and updates his status on Facebook to "in severe pain". Nelson replies, posting "Haw Haw!".
In "Team Homer", many of the kids were forgetting their trademark characteristics and when he saw Bart unsure on what do with a slingshot, he said "Ha Ho?".
Sick (Version 1)
Sick (Version 2)
In "Bart Carny", Bart and Lisa open their back door dressed in a silly way. Nelson says "Ha" before they close the door. When the door is opened again, Nelson says the second "Ha!"
In "Little Girl in the Big Ten", Nelson laughs at Bart when he is forced to live in a plastic bubble to avoid spreading his illness to others. As Bart walks, Nelson runs from one streetlight to another and laughs again, this time slow and intercepted with panting since he is out of breath.
In "'Scuse Me While I Miss the Sky", Nelson attempts his laugh, but it comes out slightly off-sounding. He then breathes into a tuning breather, and laughs the usual way.
In "The Fat and the Furriest", Nelson's arms are tied up after he tries to shoplift at Sprawl-Mart. However, he manages to shoplift by sucking up products with his mouth. After doing this, he attempts his laugh while his mouth is still full.
The Human Donkey
In "Treehouse of Horror XXIV", Nelson is a donkey satyr who wears tattered clothes and his laugh sounds like a donkey braying which is "Hee-Haw!"
In "Thanksgiving of Horror", Nelson is a turkey who says "Gobble, Gobble!"
Uses by Other Characters of Simpsons
- Mrs. Muntz has been known to use the laugh.
- Nelson's son in the future also uses the laugh, though this occurrence isn't considered canon.
- Cypress Hill member B-Real says "Ha-ha!" when he overhears Marge referring to Bart wetting his pants at the Christmas play. 
- Lisa also says it in the episode "The Haw-Hawed Couple" after discovering that Bart and Nelson are best friends.
- Nelson laughs at a tall man driving a Volkswagen Beetle, who turns the tables on him. The man makes Nelson walk down the street with his pants pulled down around his ankles, while everyone in Springfield laughs at him.
- In "Who Shot Mr. Burns? (Part One)", Nelson laughed at Santa's Little Helper's misfortune at the Vet's, his parrot also used the laugh.
- In "A Milhouse Divided", Nelson mentions the divorce of Milhouse's parents. Milhouse imitates the laugh asking whether Nelson will use it; instead, however, Nelson gives Milhouse reassurance about his difficult situation.
- In "Grift of the Magi", Funzo says "Ha-ha!" presumably because the toy company heard Nelson saying it.
- In "HOMЯ", after the newly intelligent Homer tricks Nelson into embarrassing himself in the Springfield Elementary School auditorium, the kids in the auditorium all say "Ha-ha!" to him.
- An unidentified newspaper sales kid utters the laugh shortly after conning a guy into buying a newspaper that he claimed had the Be Sharps revival (when in reality it doesn't mention anything about the Be Sharps at all, much less their revival), and then being chased by the guy.
- In "The Father, the Son and the Holy Guest Star", Edna Krabappel voices the laugh after assigning Nelson with a role in a performance that he doesn't want.
- In "Treehouse of Horror XVI", Rabbit form Apu says "Ha-Ha! You can't kill a Hindu!" Before getting stuck in a bear trap.
- In "Thanksgiving of Horror", Millhouse says it after the jelly monster eats Nelson's bones.
- ↑ I, (Annoyed Grunt)-Bot
- ↑ 2.0 2.1 The Simpsons Movie
- ↑ Grade School Confidential
- ↑ El Viaje Misterioso de Nuestro Jomer (The Mysterious Voyage of Homer)
- ↑ Margical History Tour
- ↑ Boy-Scoutz 'n the Hood
- ↑ Rome-Old and Julie-Eh
- ↑ Boy Meets Curl
- ↑ In the Name of the Grandfather
- ↑ The Debarted
- ↑ "Loan-a Lisa"
- ↑ Lisa's Wedding
- ↑ Homerpalooza
- ↑ 22 Short Films About Springfield
- ↑ "Homer's Barbershop Quartet"