The sweet-sounding name belongs to an Ecstasy-like designer drug that is being blamed for the deaths of Olivia Rotondo, 20 and Jeffery Russ, 23, during the Electric Zoo music festival.
Four others landed in intensive care and the three-day event -- which attracts tens of thousands to see artists such as David Guetta -- was cancelled ahead of its final day on Sunday "due to serious health risks," according to the mayor's office.
A New York Police Department spokeswoman told AFP the deaths appeared to be related to "a narcotic commonly known as Molly."
Molly is thought by many users to be a "pure" form of MDMA or Ecstasy, an hallucinogenic party drug which is often laced with dangerous substances.
"There is nothing pure about it at all," said Erin Mulvey, spokeswoman for the Drug Enforcement Agency's New York division.
She told AFP the drug was entirely synthetic, based on a stimulant called methylone which enters the country from places such as Canada, China or India and is turned into capsules or powder form to be snorted or ingested.
The party drug of choice for this dance generation, Molly has increasingly come under the spotlight in the past year, surging into popular culture through the mouths of some celebrities.
Madonna sparked outrage after she took to the stage at Miami's Ultra Music Festival last year and said: "How many people in this crowd have seen Molly?"
She later said she was referring to a friend's song. However that song refers to a girl named Molly who makes him "want to dance."
Hip-hop star Kanye West sings about it, as does Miley Cyrus who had the lyric "dancing with Molly" bleeped out of her performance of "We Can't Stop" at the MTV music video awards last Sunday.
"It is kind of like an endorsement of anything else. If Madonna and Miley know about it, I should know about it," said Robert Thompson, pop culture expert at the University of Syracuse
He likened the innocent-sounding name to nicknames used by previous generations such as Mary-Jane for marijuana or Lucy in the Sky with Diamonds for the psychedelic drug LSD.
The cryptic code-name has become a kind of inside joke.
One photo circulating from the Electric Zoo festival shows five friends in neon yellow tops each spelling out a different letter of the word "Molly'
It has also become a veiled way for youngsters to boast about drug abuse on social media.
"Kids are saying on Twitter: 'I am going to see Molly tonight', their parents have no idea who Molly is," said Mulvey of the DEA.
She said the harmless name was just another way distributors were trying to make their product enticing to teenagers and college kids.
"We have seen Molly in a pink crystalized form looking like candy," said Mulvey, warning that despite its reputation as a less damaging drug "there is nothing safe about it at all."
"It is a stimulant with very detrimental effects, your temperature increases, you are almost burning inside. Blood pressure increases, you can go into a coma."
"People assume it is going to be a euphoric event and they end up in the emergency room. There have been overdoses throughout the entire United States."
Yet while Molly may be getting more popular, Ellen Borakove, spokeswoman for the New York chief medical examiner, said that there were "a very small number" of deaths from the drug.
She highlighted that drug overdoses were "very often a combination of more than one drug."
After the Electric Zoo deaths, Molly's image has become a little bit less conspiratorial and hush-hush.
One Twitter user commented: "Last night my mom was watching the news and asked me if I knew who Molly was hahahahaha."