When you are in Mexico (or watch Mexican novelas) you will hear this word a lot. Seems to be no logic behind its meaning, so it might take a while to understand what we Mexicans mean when we say this.
I have two answers. One short and the other looooong.
The short one. Chilango is an unofficial but widely spread denonym for Mexico City locals.
The long one. Since the term Mexicans is already taken by 138 million locals in the country, what was left to the denizens of Mexico City? Mexico Citizens? Mexiquenians? Please consider that Mexiquenses is a term saved by the Mexico State inhabitants. Indeed, another State whose capital is Toluca and not Mexico City.
For years, the official denonym was Defeños (locals of District Federal, or D.F.) or Capitalinos (residentes of the capital). However, in 2016, a Constitutional reform disappeared D.F. and instead created the 32nd state whose name was only Ciudad de México (City of Mexico, officially in English), or CDMX for short. And, yes, they were capitalinos, but also the inhabitants to the other 31 state capitals in the country.So, how to call not only the 9 million people living in CDMX but the remaining 11 million living in the metro area within Mexico State boundaries?
And the term Chilango miraculously popped-out. In fact, Chilango is a word that has been used for centuries. No one really knows its origins and there are so many theories as Chilangos live in CDMX.
One of the most documented theories is offered by Luis Cabrera is his Diccionario de Aztequismos. Chilango is a deviation from the Nahuatl word xilaan (pr. she-laan) meaning “messy hair”. The theory goes saying that these word, xilaan, was used in Veracruz port to name anyone coming from inside the country and not the coast. Their most notorious characteristic was that they had fuzzy hair, because of lack of water to bath. So people from Puebla, Orizaba and Mexico City were naturally called xilaan.
As the word evolved into chilango, it was used more as a derogatory term only for Mexico City locals. I still remember when I was a kid a common phrase in North Mexico: “Haz patria, mata un chilango” (Make the Motherland happy, kill a Chilango). You know, this very special and loving feeling that people not living in the big city have against those living the Big Life.
However, just like gringo, it has lost most of that derogatory in the last few years. Now there is even a hip magazine published in CDMX called Chilango with the best recommendations of Mexico City’s style (a somehow posh and aspirational print). Chilango has turned into a word used by Chilangos themselves and has reached major media outlets devoid of its previous derogatory sense. So when, they were no longer able to be called Defeños, Chilangos was the top choice. And yes, the city they live in is also dubbed Chilangolandia.
Though it is mostly used to name people living in Mexico City and its metro area, a wider use would include anyone with Chilango accent. Yep, more confusion to the equation.
Every region in Mexico has a distinct Spanish accent, quite different among themselves, and obviously different from the Iberian, Colombia, Cuban, Peruvian or Argentinian accent. Central Mexico has this thick and singing tone quite recognizable. In fact, due to the TV shows produced in Mexico City, in many Latin American countries people will think it is the standard Mexican accent. So, all people living in Central Mexico and sharing Mexico City accent are sometimes also grouped as Chilangos.
See? I should have sticked to the short answer: locals of Mexico City (CDMX)