That comes from "huevón", which is... really hard to explain in English. The Argentine "boludo" and "pelotudo" are in the same vein, and mean roughly the same non-sensical thing.
I'm not sure if the etymology is that an egg ("huevo") looks like a zero, so a dumb person or a loser is a "huevo" (an egg, meaning a zero) or "huevón" (huge zero), or if it has to do with testicles (because in Spanish it's common to call them eggs), so a "huevón" might someone who's "as dumb as a giant testicle". "Boludo" and "pelotudo" are more like the latter, for sure, though they could still derived from zero looking like a ball.
> The "w" sound keeps repeating in the themes here, interesting.
In weón it's really just a slight change from huevón, and mostly just dropping the 'v'.