Chloe Swarbrick told an older lawmaker "OK boomer" after they interrupted her speech on climate change, the BBC reported.
A "boomer" is shorthand for a baby boomer - someone born between 1946 and 1964.
In Internet parlance, "OK boomer" is a derogatory phrase used primarily by the next generations to show their indignation towards older people deemed indifferent to their concerns. It is used widely on platforms like Twitter and TikTok.
"Boomer is a state of mind," the BBC quoted Swarbrick as telling local media.
"I think you can see from the way that that meme has evolved that it is symbolic of the collective frustration that young people in particular feel to placing evidence in fact time after time in the debate and in the argument and being met with dogma," she said.
Swarbrick was commenting on the Zero Carbon bill, which aims to reduce net carbon emissions in New Zealand to zero by 2050, when she used the phrase.
"Mr Speaker, how many world leaders, for how many decades have seen and known what is coming but have decided that it is more politically expedient to keep it behind closed doors. My generation and the generations after me do not have that luxury," she said.
"In the year 2050, I will be 56 years old. Yet, right now, the average age of this 52nd Parliament is 49 years old."
At this point in her speech, she was interrupted by an older MP, reported to be opposition spokesman for climate change, Todd Muller.
Swarbrick paused, gestured with her right hand and said: "OK boomer."
On social media, the Green MP - elected in 2017 - has been hailed as a "queen" for using the term. But some critics see "OK boomer" as ageist.
Fellow New Zealand lawmaker Christopher Bishop expressed that "unpopular and non-woke opinion" in a tweet.
Muller, on the other hand, wondered how long Swarbrick would remain a "millennial force for change".
( With inputs from IANS )