"weon" is a corruption of the word "huevón", that means literally "big egg".
Is a polysemic word, but when used as adjective means "somebody with big balls"; by extension "somebody that spends the day sitting on their own testicles, unable to carry them"
So a huevón is "a lazy lad", "a douchebad" or simply "a dude" (colloquially and vulgar, but also playful if applied to a close friend).
The word is a minefield, some people will feel amused, other insulted, and is a faux pas with women. Better avoid it unless you know what you are doing.
The real value of this words are not in their literal meaning. Is an interesting question, in fact
When Chileans drop 'weon' ten times in two minutes in their conversations what they are saying is "I'm not Argentinian".
This is the real message.
Because Argentinians would use their trade mark "boludo" instead. Boludo is exactly the same that Weon. A boludo is somebody with "big balls". A weon is somebody with "big eggs". Both words have small differences in their meanings but are mutually exclusive (You either use one or the other). They are tribal tags.
If you want to do business with a Chilean, knowing were to casually drop 'weon' in an --strictly-- informal context is pure gold. It grants instant ghetto pass. Mexicans or Argentinians have their own equivalent words. If you use the wrong password the doors will close, so is risky and you need to dominate the context and use it sparingly. The overuse could be socially awkward and outdated (Imagine somebody from Sevilla going to Texas dressed on a fake cowboy disguise, pretending to walk like one and talking to you about business while spitting tobacco and flashing a weapon. Is 120% stoopid).
Spanish world is complex. Latinos can belong to several worlds at the same time. Lets take for example the singer Gloria Stephan. Gloria Stephan is a 100% USA citizen. But it was born in Cuba so felts Cuban traditions also as part of her culture. And her Grandparents are from the North of Spain, so Gloria Stephan identity is an USA-Cuban-Spanish-almost-Irish music star.
Most Latinos learn soon to adjust their different identities to the context appropriate, and wear or drop their accents and languages accordingly.
On the other hand, when somebody choose to say 'mergear' instead 'mezclar', the message for millions of people is: "look ma, I'm talking English. Almost". As the band Blur would say: "He'd like to live in magic America with all those magic people". Youngsters crave for acceptation and local friendship, so Spanglish or local accents are seen as something very valuable for making relationships
But grown people needs to adjust the message later. If you are a Chilean that migrate to Spain and keep saying "weon" all the time, what you are remembering to everybody is "I don't belong there".
And is the same problem with Spanglish. If you want to use English, by Pete's sake, talk in the most correct English that you can afford. Get the respect that you deserve, instead to introduce yourself like a wannabee latino redneck. If you need Spanish in business, the safe baby-proof choice is to learn neuter Spanish. Period. Will provide the most bang for the buck. But don't be ashamed to wear the multiple rich cloaks of Spanish in your benefit in the correct contexts.
"el weon weon, weon."